copyright © 2011 pearson education, inc. chapter 4 chemical quantities and aqueous reactions roy...
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Chapter 4Chemical Quantities
and Aqueous Reactions
Roy KennedyMassachusetts Bay Community College
Wellesley Hills, MA
Chemistry: A Molecular Approach, 2nd Ed.Nivaldo Tro
Copyright © 2011 Pearson Education, Inc.
Global Warming
• Scientists have measured an average 0.6 °C rise in atmospheric temperature since 1860
• During the same period atmospheric CO2 levels have risen 25%
• Are the two trends causal?
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The Sources of Increased CO2
• One source of CO2 is the combustion reactions of fossil fuels we use to get energy
• Another source of CO2 is volcanic action• How can we judge whether global warming is natural or
due to our use of fossil fuels?
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Quantities in Chemical Reactions
• The amount of every substance used and made in a chemical reaction is related to the amounts of all the other substances in the reactionLaw of Conservation of MassBalancing equations by balancing atoms
• The study of the numerical relationship between chemical quantities in a chemical reaction is called stoichiometry
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Reaction Stoichiometry
• The coefficients in a balanced chemical equation specify the relative amounts in moles of each of the substances involved in the reaction2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g)
2 molecules of C8H18 react with 25 molecules of O2
to form 16 molecules of CO2 and 18 molecules of H2O
2 moles of C8H18 react with 25 moles of O2
to form 16 moles of CO2 and 18 moles of H2O
2 mol C8H18 : 25 mol O2 : 16 mol CO2 : 18 mol H2O
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Making Pizza
• The number of pizzas you can make depends on the amount of the ingredients you use
• This relationship can be expressed mathematically1 crust : 5 oz. sauce : 2 cu cheese : 1 pizza
1 crust + 5 oz. tomato sauce + 2 cu cheese 1 pizza
• If you want to make more or less than one pizza, you can use the amount of cheese you have to determine the number of pizzas you can makeassuming you have enough crusts and tomato sauce
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Predicting Amounts from Stoichiometry
• The amounts of any other substance in a chemical reaction can be determined from the amount of just one substance
• How much CO2 can be made from 22.0 moles of C8H18 in the combustion of C8H18?
2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g)
2 moles C8H18 : 16 moles CO2
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Practice
• According to the following equation, how many moles of water are made in the combustion of 0.10 moles of glucose?
C6H12O6 + 6 O2 6 CO2 + 6 H2O
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Practice How many moles of water are made in the combustion of 0.10 moles of glucose?
0.6 mol H2O = 0.60 mol H2Obecause 6x moles of H2O as C6H12O6, the number makes
sense
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O 1 mol C6H12O6 : 6 mol H2O
0.10 moles C6H12O6
moles H2O
Check:
Solution:
Conceptual Plan:
Relationships:
Given:Find:
mol H2Omol C6H12O6
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Example: Estimate the mass of CO2 produced in 2007 by the combustion of 3.5 x 1015 g gasolne
• Assuming that gasoline is octane, C8H18, the equation for the reaction is
2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g)
• The equation for the reaction gives the mole relationship between amount of C8H18 and CO2, but we need to know the mass relationship, so the conceptual plan will be
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Example: Estimate the mass of CO2 produced in 2007 by the combustion of 3.5 x 1015 g gasoline
because 8x moles of CO2 as C8H18, but the molar mass of C8H18 is 3x CO2, the number makes sense
1 mol C8H18 = 114.22g, 1 mol CO2 = 44.01g, 2 mol C8H18:16 mol CO2
3.4 x 1015 g C8H18
g CO2
Check:
Solution:
Conceptual Plan:
Relationships:
Given:Find:
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Which Produces More CO2; Volcanoes or Fossil Fuel Combustion?• Our calculation just showed that the world
produced 1.1 x 1016 g of CO2 just from petroleum combustion in 20071.1 x 1013 kg CO2
• Estimates of volcanic CO2 production are 2 x 1011 kg/year
• This means that volcanoes produce less than 2% of the CO2 added to the air annually
12
%.%.
.81100
1011
1002
yrkg13
yrkg11
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Example 4.1: How many grams of glucose can be synthesized from 37.8 g of CO2 in photosynthesis?
because 6x moles of CO2 as C6H12O6, but the molar mass of C6H12O6 is 4x CO2, the number makes sense
1 mol C6H12O6 = 180.2g, 1 mol CO2 = 44.01g, 1 mol C6H12O6 : 6 mol CO2
37.8 g CO2, 6 CO2 + 6 H2O C6H12O6+ 6 O2
g C6H12O6
Check:
Solution:
Conceptual Plan:
Relationships:
Given:Find:
g 44.01mol 1
6126
6126
6126
2
6126
2
22
OHC g 5.82 OHC mol 1
OHC g 180.2
CO mol 6
OHC mol 1
CO g 44.01
CO mol 1CO g 7.83
2
6126
CO mol 6OHC mol 1
mol 1
g 180.2
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Practice — How many grams of O2 can be made from the decomposition of 100.0 g of PbO2?
2 PbO2(s) → 2 PbO(s) + O2(g)(PbO2 = 239.2, O2 = 32.00)
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Practice — How many grams of O2 can be made from the decomposition of 100.0 g of PbO2?
2 PbO2(s) → 2 PbO(s) + O2(g)
because ½ moles of O2 as PbO2, and the molar mass of PbO2 is 7x O2, the number makes sense
1 mol O2 = 32.00g, 1 mol PbO2 = 239.2g, 1 mol O2 : 2 mol PbO2
100.0 g PbO2, 2 PbO2 → 2 PbO + O2
g O2
Check:
Solution:
Conceptual Plan:
Relationships:
Given:Find:
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Stoichiometry Road Mapa A b B
Moles A Moles B
mass A mass B
volume A (l) volume B(l)
Pur
eS
ubst
ance
Sol
utio
n
Volume A(g) Volume B(g)
% A(aq)ppm A(aq)
% B(aq)ppm B(aq)
M A(aq) M B(aq)
MM MM
density density
equation
equation
22.4 L 22.4 L
M = molesL
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More Making Pizzas
17
• We know that
1 crust + 5 oz. tomato sauce + 2 cu cheese 1 pizza
• But what would happen if we had 4 crusts,
15 oz. tomato sauce, and 10 cu cheese?
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More Making Pizzas, Continued• Each ingredient could potentially make a
different number of pizzas
• But all the ingredients have to work together!
• We only have enough tomato sauce to make three pizzas, so once we make three pizzas, the tomato sauce runs out no matter how much of the other ingredients we have.
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More Making Pizzas, Continued
• The tomato sauce limits the amount of pizzas we can make. In chemical reactions we call this the limiting reactant.also known as the limiting reagent
• The maximum number of pizzas we can make depends on this ingredient. In chemical reactions, we call this the theoretical yield. it also determines the amounts of the other
ingredients we will use!
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The Limiting Reactant• For reactions with multiple reactants, it is likely that one
of the reactants will be completely used before the others
• When this reactant is used up, the reaction stops and no more product is made
• The reactant that limits the amount of product is called the limiting reactantsometimes called the limiting reagent the limiting reactant gets completely consumed
• Reactants not completely consumed are called excess reactants
• The amount of product that can be made from the limiting reactant is called the theoretical yield
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Limiting and Excess Reactants in the Combustion of Methane
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)
• Our balanced equation for the combustion of methane implies that every one molecule of CH4 reacts with two molecules of O2
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Limiting and Excess Reactants in the Combustion of Methane
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)
since less CO2 can be made from the O2 than the CH4, so the O2 is the limiting reactant
22
• If we have five molecules of CH4 and eight molecules of O2, which is the limiting reactant?
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Practice — How many moles of Si3N4 can be made from 1.20 moles of Si and 1.00 moles
of N2 in the reaction 3 Si + 2 N2 Si3N4?
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Limitingreactant
Practice — How many moles of Si3N4 can be made from 1.20 moles of Si and 1.00 moles of N2 in the reaction
3 Si + 2 N2 Si3N4?
2 mol N2 : 1 Si3N4; 3 mol Si : 1 Si3N4
1.20 mol Si, 1.00 mol N2
mol Si3N4
Solution:
Conceptual Plan:
Relationships:
Given:Find:
Theoretical yield
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More Making Pizzas• Let’s now assume that as we are making
pizzas, we burn a pizza, drop one on the floor, or other uncontrollable events happen so that we only make two pizzas. The actual amount of product made in a chemical reaction is called the actual yield.
• We can determine the efficiency of making pizzas by calculating the percentage of the maximum number of pizzas we actually make. In chemical reactions, we call this the percent yield.
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Theoretical and Actual Yield• As we did with the pizzas, in order to determine
the theoretical yield, we should use reaction stoichiometry to determine the amount of product each of our reactants could make
• The theoretical yield will always be the least possible amount of product the theoretical yield will always come from the limiting
reactant
• Because of both controllable and uncontrollable factors, the actual yield of product will always be less than the theoretical yield
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Example 4.4:Finding limiting reactant,
theoretical yield, and percent yield
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Example:
• When 28.6 kg of C are allowed to react with 88.2 kg of TiO2 in the reaction below, 42.8 kg of Ti are obtained. Find the limiting reactant, theoretical yield, and percent yield.
TiO2(s) 2 C(s) Ti(s) 2 CO(g)
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Example:When 28.6 kg of C reacts with 88.2 kg of TiO2, 42.8 kg of Ti are obtained. Find the limiting reactant, theoretical yield, and percent yield TiO2(s) + 2 C(s) Ti(s) + 2 CO(g)
• Write down the given quantity and its units
Given: 28.6 kg C
88.2 kg TiO2
42.8 kg Ti produced
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Example:Find the limiting reactant, theoretical yield, and percent yield
TiO2(s) + 2 C(s) Ti(s) + 2 CO(g)
• Write down the quantity to find and/or its units
Find: limiting reactant
theoretical yield
percent yield
InformationGiven: 28.6 kg C, 88.2 kg TiO2, 42.8 kg Ti
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Example:Find the limiting reactant, theoretical yield, and percent yield TiO2(s) + 2 C(s) Ti(s) + 2 CO(g)
• Write a conceptual plan
Information
Given: 28.6 kg C, 88.2 kg TiO2, 42.8 kg Ti
Find: lim. rct., theor. yld., % yld.
kgTiO2
kgC }
smallestamount is
from limitingreactant
smallestmol Ti
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Example:Find the limiting reactant, theoretical yield, and percent yield TiO2(s) + 2 C(s) Ti(s) + 2 CO(g)
• Collect needed relationships
1000 g = 1 kg
Molar Mass TiO2 = 79.87 g/mol
Molar Mass Ti = 47.87 g/mol
Molar Mass C = 12.01 g/mol
1 mole TiO2 : 1 mol Ti (from the chem. equation)
2 mole C : 1 mol Ti (from the chem. equation)
Information
Given: 28.6 kg C, 88.2 kg TiO2, 42.8 kg Ti
Find: lim. rct., theor. yld., % yld.
CP: kg rct g rct mol rct mol Ti
pick smallest mol Ti TY kg Ti %Y Ti
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• Apply the conceptual plan
InformationGiven: 28.6 kg C, 88.2 kg TiO2, 42.8 kg Ti Find: lim. rct., theor. yld., % yld.CP: kg rct g rct mol rct mol Tipick smallest mol Ti TY kg Ti %Y Ti Rel: 1 mol C=12.01g; 1 mol Ti =47.87g; 1 mol TiO2 = 79.87g; 1000g = 1 kg; 1 mol TiO2 : 1 mol Ti; 2 mol C : 1 mol Ti
Example:Find the limiting reactant, theoretical yield, and percent yield TiO2(s) + 2 C(s) Ti(s) + 2 CO(g)
smallest moles of Tilimiting reactant
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• Apply the conceptual plan
theoretical yield
InformationGiven: 28.6 kg C, 88.2 kg TiO2, 42.8 kg Ti Find: lim. rct., theor. yld., % yld.CP: kg rct g rct mol rct mol Tipick smallest mol Ti TY kg Ti %Y Ti Rel: 1 mol C=12.01g; 1 mol Ti =47.87g; 1 mol TiO2 = 79.87g; 1000g = 1 kg; 1 mol TiO2 : 1 mol Ti; 2 mol C : 1 mol Ti
Example:Find the limiting reactant, theoretical yield, and percent yield TiO2(s) + 2 C(s) Ti(s) + 2 CO(g)
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• Apply the conceptual plan
InformationGiven: 28.6 kg C, 88.2 kg TiO2, 42.8 kg Ti Find: lim. rct., theor. yld., % yld.CP: kg rct g rct mol rct mol Tipick smallest mol Ti TY kg Ti %Y Ti Rel: 1 mol C=12.01g; 1 mol Ti =47.87g; 1 mol TiO2 = 79.87g; 1000g = 1 kg; 1 mol TiO2 : 1 mol Ti; 2 mol C : 1 mol Ti
Example:Find the limiting reactant, theoretical yield, and percent yield TiO2(s) + 2 C(s) Ti(s) + 2 CO(g)
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• Check the solutions
limiting reactant = TiO2
theoretical yield = 52.9 kgpercent yield = 80.9%
Because Ti has lower molar mass than TiO2, the T.Y. makes sense and the percent yield makes sense as it is less than 100%
InformationGiven: 28.6 kg C, 88.2 kg TiO2, 42.8 kg Ti Find: lim. rct., theor. yld., % yld.CP: kg rct g rct mol rct mol Tipick smallest mol Ti TY kg Ti %Y Ti Rel: 1 mol C=12.01g; 1 mol Ti =47.87g; 1 mol TiO2 = 79.87g; 1000g = 1 kg; 1 mol TiO2 : 1 mol Ti; 2 mol C : 1 mol Ti
Example:Find the limiting reactant, theoretical yield, and percent yield TiO2(s) + 2 C(s) Ti(s) + 2 CO(g)
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Practice — How many grams of N2(g) can be made from 9.05 g of NH3 reacting with 45.2 g of CuO?
2 NH3(g) + 3 CuO(s) → N2(g) + 3 Cu(s) + 3 H2O(l)If 4.61 g of N2 are made, what is the percent yield?
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Practice — How many grams of N2(g) can be made from 9.05 g of NH3 reacting with 45.2 g of CuO? 2 NH3(g) + 3 CuO(s) → N2(g) + 3 Cu(s) + 3 H2O(l)
If 4.61 g of N2 are made, what is the percent yield?
1 mol NH3 = 17.03g, 1 mol CuO = 79.55g, 1 mol N2 = 28.02 g2 mol NH3 : 1 mol N2, 3 mol CuO : 1 mol N2
9.05 g NH3, 45.2 g CuOg N2
Conceptual Plan:
Relationships:
Given:Find:
Choose smallest
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because the percent yield is less than 100, the answer makes sense
Check:
Solution:
Theoreticalyield
39
Practice — How many grams of N2(g) can be made from 9.05 g of NH3 reacting with 45.2 g of CuO? 2 NH3(g) + 3 CuO(s) → N2(g) + 3 Cu(s) + 3 H2O(l)
If 4.61 g of N2 are made, what is the percent yield?
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Solutions• When table salt is mixed with water, it seems to
disappear, or become a liquid – the mixture is homogeneous the salt is still there, as you can tell from the taste, or simply
boiling away the water
• Homogeneous mixtures are called solutions
• The component of the solution that changes state is called the solute
• The component that keeps its state is called the solvent if both components start in the same state, the major
component is the solvent
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Describing Solutions
• Because solutions are mixtures, the composition can vary from one sample to anotherpure substances have constant compositionsaltwater samples from different seas or lakes have
different amounts of salt
• So to describe solutions accurately, we must describe how much of each component is presentwe saw that with pure substances, we can describe
them with a single name because all samples are identical
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Solution Concentration
• Qualitatively, solutions are often described as dilute or concentrated
• Dilute solutions have a small amount of solute compared to solvent
• Concentrated solutions have a large amount of solute compared to solvent
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Concentrations—Quantitative Descriptions of Solutions
• A more precise method for describing a solution is to quantify the amount of solute in a given amount of solution
• Concentration = amount of solute in a given amount of solutionoccasionally amount of solvent
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Solution ConcentrationMolarity
• Moles of solute per 1 liter of solution
• Used because it describes how many molecules of solute in each liter of solution
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Preparing 1 L of a 1.00 M NaCl Solution
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Example 4.5: Find the molarity of a solution that has 25.5 g KBr dissolved in 1.75 L of solution
because most solutions are between 0 and 18 M, the answer makes sense
1 mol KBr = 119.00 g, M = moles/L
25.5 g KBr, 1.75 L solutionmolarity, M
Check:
Solution:
Conceptual Plan:
Relationships:
Given:Find:
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Practice — What Is the molarity of a solution containing 3.4 g of NH3 (MM 17.03) in 200.0 mL of solution?
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Practice — What Is the molarity of a solution containing 3.4 g of NH3 (MM 17.03) in 200.0 mL of solution?
the unit is correct, the number is reasonable because the fraction of moles is less than the fraction of liters
Check:
Solve:
M = mol/L, 1 mol NH3 = 17.03 g, 1 mL = 0.001 L
Conceptual Plan:
Relationships:
3.4 g NH3, 200.0 mL solution
M
Given:
Find:
g NH3 mol NH3
mL sol’n L sol’nM
48
0.20 mol NH3, 0.2000 L solution
M
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Using Molarity in Calculations
• Molarity shows the relationship between the moles of solute and liters of solution
• If a sugar solution concentration is 2.0 M, then 1 liter of solution contains 2.0 moles of sugar 2 liters = 4.0 moles sugar 0.5 liters = 1.0 mole sugar
• 1 L solution : 2 moles sugar
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Example 4.6: How many liters of 0.125 M NaOH contain 0.255 mol NaOH?
because each L has only 0.125 mol NaOH, it makes sense that 0.255 mol should require a little more than 2
L
0.125 mol NaOH = 1 L solution
0.125 M NaOH, 0.255 mol NaOHliters, L
Check:
Solution:
Conceptual Plan:
Relationships:
Given:Find:
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Practice — Determine the mass of CaCl2
(MM = 110.98) in 1.75 L of 1.50 M solution
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Practice — Determine the mass of CaCl2
(MM = 110.98) in 1.75 L of 1.50 M solution
because each L has 1.50 mol CaCl2, it makes sense that 1.75 L should have almost 3 moles
1.50 mol CaCl2 = 1 L solution; 110.98 g CaCl2 = 1 mol
1.50 M CaCl2, 1.75 Lmass CaCl2, g
Check:
Solution:
Conceptual Plan:
Relationships:
Given:Find:
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Example: How would you prepare 250.0 mL of a 1.00 M solution CuSO45 H2O(MM 249.69)?
the unit is correct, the magnitude seems reasonable as the volume is ¼ of a liter
Check:
Solution:
1.00 L sol’n = 1.00 mol; 1 mL = 0.001 L; 1 mol = 249.69 g
Conceptual Plan:
Relationships:
250.0 mL solution
mass CuSO4 5 H2O, g
Given:
Find:
Dissolve 62.4 g of CuSO4∙5H2O in enough water to total 250.0 mL
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Practice – How would you prepare 250.0 mL of 0.150 M CaCl2 (MM = 110.98)?
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Practice – How would you prepare 250.0 mL of 0.150 M CaCl2?
the unit is correct, the magnitude seems reasonable as the volume is ¼ of a liter
Check:
Solution:
1.00 L sol’n = 0.150 mol; 1 mL = 0.001L; 1 mol = 110.98 g
Conceptual Plan:
Relationships:
250.0 mL solution
mass CaCl2, g
Given:
Find:
Dissolve 4.16 g of CaCl2 in enough water to total 250.0 mL
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Dilution
• Often, solutions are stored as concentrated stock solutions
• To make solutions of lower concentrations from these stock solutions, more solvent is added the amount of solute doesn’t change, just the volume of
solution
moles solute in solution 1 = moles solute in solution 2
• The concentrations and volumes of the stock and new solutions are inversely proportional
M1∙V1 = M2∙V2
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Example 4.7: To what volume should you dilute 0.200 L of 15.0 M NaOH to make 3.00 M NaOH?
because the solution is diluted by a factor of 5, the volume should increase by a factor of 5, and it does
M1V1 = M2V2
V1 = 0.200L, M1 = 15.0 M, M2 = 3.00 MV2, L
Check:
Solution:
Conceptual Plan:
Relationships:
Given:Find:
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Practice – What is the concentration of a solution prepared by diluting 45.0 mL of
8.25 M HNO3 to 135.0 mL?
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Practice – What is the concentration of a solution prepared by diluting 45.0 mL of 8.25 M HNO3 to 135.0 mL?
because the solution is diluted by a factor of 3, the molarity should decrease by a factor of 3, and it does
M1V1 = M2V2
V1 = 45.0 mL, M1 = 8.25 M, V2 = 135.0 mLM2, L
Check:
Solution:
Conceptual Plan:
Relationships:
Given:Find:
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Practice – How would you prepare 200.0 mL of 0.25 M NaCl solution from a 2.0 M solution?
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Practice – How would you prepare 200.0 mL of 0.25 M NaCl solution from a 2.0 M solution?
because the solution is diluted by a factor of 8, the volume should increase by a factor of 8, and it does
M1V1 = M2V2
M1 = 2.0 M, M2 = 0.25 M, V2 = 200.0 mLV1, L
Check:
Solution:
Conceptual Plan:
Relationships:
Given:Find:
Dilute 25 mL of 2.0 M solution up to 200.0 mL
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Solution Stoichiometry
• Because molarity relates the moles of solute to the liters of solution, it can be used to convert between amount of reactants and/or products in a chemical reaction
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Example 4.8: What volume of 0.150 M KCl is required to completely react with 0.150 L of 0.175 M Pb(NO3)2 in the
reaction 2 KCl(aq) + Pb(NO3)2(aq) PbCl2(s) + 2 KNO3(aq)?
because you need 2x moles of KCl as Pb(NO3)2, and the molarity of Pb(NO3)2 > KCl, the volume of KCl should be
more than 2x the volume of Pb(NO3)2
1 L Pb(NO3)2 = 0.175 mol, 1 L KCl = 0.150 mol, 1 mol Pb(NO3)2 : 2 mol KCl
0.150 M KCl, 0.150 L of 0.175 M Pb(NO3)2
L KCl
Check:
Solution:
Conceptual Plan:
Relationships:
Given:
Find:
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Practice — Solution stoichiometry• 43.8 mL of 0.107 M HCl is needed to
neutralize 37.6 mL of Ba(OH)2 solution. What is the molarity of the base?
2 HCl + Ba(OH)2 BaCl2 + 2 H2O
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Practice – 43.8 mL of 0.107 M HCl is needed to neutralize 37.6 mL of Ba(OH)2 solution. What is the molarity of the
base? 2 HCl(aq) + Ba(OH)2(aq) BaCl2(aq) + 2 H2O(aq)
the units are correct, the number makes sense because there are fewer moles than liters
1 mL= 0.001 L, 1 L HCl = 0.107 mol, 1 mol Ba(OH)2 : 2 mol HCl
43.8 mL of 0.107 M HCl, 37.6 mL Ba(OH)2
M Ba(OH)2
Check:
Solution:
Conceptual Plan:
Relationships:
Given:
Find:
L Ba(OH)2
M Ba(OH)2
0.0438 L of 0.107 M HCl, 0.0376 L Ba(OH)2
M Ba(OH)2
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What Happens When a Solute Dissolves?• There are attractive forces between the solute particles
holding them together• There are also attractive forces between the solvent
molecules• When we mix the solute with the solvent, there are
attractive forces between the solute particles and the solvent molecules
• If the attractions between solute and solvent are strong enough, the solute will dissolve
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Table Salt Dissolving in WaterEach ion is attracted to the surrounding water molecules and pulled off and away from the crystal
When it enters the solution, the ion is surrounded by water molecules, insulating it from other ions
The result is a solution with free moving charged particles able to conduct electricity
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Electrolytes and Nonelectrolytes• Materials that dissolve
in water to form a solution that will conduct electricity are called electrolytes
• Materials that dissolve in water to form a solution that will not conduct electricity are called nonelectrolytes
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Molecular View of Electrolytes and Nonelectrolytes
• To conduct electricity, a material must have charged particles that are able to flow
• Electrolyte solutions all contain ions dissolved in the water ionic compounds are electrolytes because they
dissociate into their ions when they dissolve
• Nonelectrolyte solutions contain whole molecules dissolved in the watergenerally, molecular compounds do not ionize when
they dissolve in water the notable exception being molecular acids
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Salt vs. Sugar Dissolved in Water
70Tro: Chemistry: A Molecular Approach, 2/e
ionic compounds dissociate into ions when
they dissolve
molecular compounds do not dissociate when
they dissolve
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Acids• Acids are molecular compounds that ionize when they
dissolve in water the molecules are pulled apart by their attraction for the waterwhen acids ionize, they form H+ cations and also anions
• The percentage of molecules that ionize varies from one acid to another
• Acids that ionize virtually 100% are called strong acidsHCl(aq) H+(aq) + Cl−(aq)
• Acids that only ionize a small percentage are called weak acids
HF(aq) H+(aq) + F−(aq)
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Strong and Weak Electrolytes
• Strong electrolytes are materials that dissolve completely as ions ionic compounds and strong acids their solutions conduct electricity well
• Weak electrolytes are materials that dissolve mostly as molecules, but partially as ionsweak acids their solutions conduct electricity, but not well
• When compounds containing a polyatomic ion dissolve, the polyatomic ion stays together
HC2H3O2(aq) H+(aq) + C2H3O2−(aq)
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Classes of Dissolved Materials
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Dissociation and Ionization• When ionic compounds dissolve in water, the
anions and cations are separated from each other. This is called dissociation.
Na2S(aq) 2 Na+(aq) + S2-(aq)
• When compounds containing polyatomic ions dissociate, the polyatomic group stays together as one ion
Na2SO4(aq) 2 Na+(aq) + SO42−(aq)
• When strong acids dissolve in water, the molecule ionizes into H+ and anions
H2SO4(aq) 2 H+(aq) + SO42−(aq)
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Practice – Write the equation for the process that occurs when the following strong electrolytes
dissolve in water
CaCl2
HNO3
(NH4)2CO3
CaCl2(aq) Ca2+(aq) + 2 Cl−(aq)
HNO3(aq) H+(aq) + NO3−(aq)
(NH4)2CO3(aq) 2 NH4+(aq) + CO3
2−(aq)
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Solubility of Ionic Compounds
• Some ionic compounds, such as NaCl, dissolve very well in water at room temperature
• Other ionic compounds, such as AgCl, dissolve hardly at all in water at room temperature
• Compounds that dissolve in a solvent are said to be soluble, where as those that do not are said to be insolubleNaCl is soluble in water, AgCl is insoluble in water the degree of solubility depends on the temperatureeven insoluble compounds dissolve, just not enough to be
meaningful
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When Will a Salt Dissolve?
• Predicting whether a compound will dissolve in water is not easy
• The best way to do it is to do some experiments to test whether a compound will dissolve in water, then develop some rules based on those experimental resultswe call this method the empirical method
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Solubility RulesCompounds that Are Generally
Soluble in Water
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Solubility RulesCompounds that Are Generally
Insoluble in Water
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Practice – Determine if each of the following is soluble in water
KOH
AgBr
CaCl2
Pb(NO3)2
PbSO4
KOH is soluble because it contains K+
AgBr is insoluble; most bromides are soluble, but AgBr is an exception
CaCl2 is soluble; most chlorides are soluble, and CaCl2 is not an exception
Pb(NO3)2 is soluble because it contains NO3−
PbSO4 is insoluble; most sulfates are soluble, but PbSO4 is an exception
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Precipitation Reactions
• Precipitation reactions are reactions in which a solid forms when we mix two solutionsreactions between aqueous
solutions of ionic compounds produce an ionic compound that
is insoluble in water the insoluble product is called a
precipitate
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2 KI(aq) + Pb(NO3)2(aq) PbI2(s) + 2 KNO3(aq)
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No Precipitate Formation = No Reaction
KI(aq) + NaCl(aq) KCl(aq) + NaI(aq)all ions still present, no reaction
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Process for Predicting the Products ofa Precipitation Reaction
1. Determine what ions each aqueous reactant has
2. Determine formulas of possible products exchange ions
(+) ion from one reactant with (-) ion from other
balance charges of combined ions to get formula of each product
3. Determine solubility of each product in water use the solubility rules if product is insoluble or slightly soluble, it will precipitate
4. If neither product will precipitate, write no reaction after the arrow
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Process for Predicting the Products ofa Precipitation Reaction
5. If any of the possible products are insoluble, write their formulas as the products of the reaction using (s) after the formula to indicate solid. Write any soluble products with (aq) after the formula to indicate aqueous.
6. Balance the equation remember to only change coefficients, not
subscripts
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Example 4.10: Write the equation for the precipitation reaction between an aqueous
solution of potassium carbonate and an aqueous solution of nickel(II) chloride
1. Write the formulas of the reactantsK2CO3(aq) + NiCl2(aq)
2. Determine the possible productsa) determine the ions present
(K+ + CO32−) + (Ni2+ + Cl−)
b) exchange the Ions
(K+ + CO32−) + (Ni2+ + Cl−) (K+ + Cl−) + (Ni2+ + CO3
2−)
c) write the formulas of the products balance charges
K2CO3(aq) + NiCl2(aq) KCl + NiCO386Tro: Chemistry: A Molecular Approach, 2/e
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Example 4.10: Write the equation for the precipitation reaction between an aqueous
solution of potassium carbonate and an aqueous solution of nickel(II) chloride
3. Determine the solubility of each productKCl is soluble
NiCO3 is insoluble
4. If both products are soluble, write no reaction
does not apply because NiCO3 is insoluble
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Example 4.10: Write the equation for the precipitation reaction between an aqueous
solution of potassium carbonate and an aqueous solution of nickel(II) chloride
5. Write (aq) next to soluble products and (s) next to insoluble products
K2CO3(aq) + NiCl2(aq) KCl(aq) + NiCO3(s)
6. Balance the equation
K2CO3(aq) + NiCl2(aq) KCl(aq) + NiCO3(s)
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Practice – Predict the products and balance the equation
KCl(aq) + AgNO3(aq) KNO3(aq) + AgCl(s)
Na2S(aq) + CaCl2(aq) NaCl(aq) + CaS(aq)No reaction
(K+ + Cl−) + (Ag+ + NO3−) → (K+ + NO3
−) + (Ag+ + Cl−)
KCl(aq) + AgNO3(aq) → KNO3 + AgCl
(Na+ + S2−) + (Ca2+ + Cl−) → (Na+ + Cl−) + (Ca2+ + S2−)
Na2S(aq) + CaCl2(aq) → NaCl + CaS
KCl(aq) + AgNO3(aq)
Na2S(aq) + CaCl2(aq)
Tro: Chemistry: A Molecular Approach, 2/e
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Practice – Write an equation for the reaction that takes place when an
aqueous solution of (NH4)2SO4 is mixed with an aqueous solution of Pb(C2H3O2)2.
(NH4)2SO4(aq) + Pb(C2H3O2)2(aq) NH4C2H3O2(aq) + PbSO4(s)
(NH4+ + SO4
2−) + (Pb2+ + C2H3O2−) → (NH4
+ + C2H3O2−) + (Pb2+ + SO4
2−)
(NH4)2SO4(aq) + Pb(C2H3O2)2(aq)
(NH4)2SO4(aq) + Pb(C2H3O2)2(aq) NH4C2H3O2 + PbSO4
90
(NH4)2SO4(aq) + Pb(C2H3O2)2(aq) 2 NH4C2H3O2(aq) + PbSO4(s)
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Ionic Equations
• Equations that describe the chemicals put into the water and the product molecules are called molecular equations
2 KOH(aq) + Mg(NO3)2(aq) 2 KNO3(aq) + Mg(OH)2(s)
• Equations that describe the material’s structure when dissolved are called complete ionic equations aqueous strong electrolytes are written as ions
soluble salts, strong acids, strong bases
insoluble substances, weak electrolytes, and nonelectrolytes are written in molecule formsolids, liquids, and gases are not dissolved, therefore molecule form
2K+(aq) + 2OH−
(aq) + Mg2+(aq) + 2NO3
−(aq) K+
(aq) + 2NO3−
(aq) + Mg(OH)2(s)
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Ionic Equations
• Ions that are both reactants and products are called spectator ions
2 K+(aq) + 2 OH−
(aq) + Mg2+(aq) + 2 NO3
−(aq) 2 K+
(aq) + 2 NO3−
(aq) + Mg(OH)2(s)
An ionic equation in which the spectator ions are removed is called a net ionic equation
2 OH−(aq) + Mg2+
(aq) Mg(OH)2(s)
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Practice – Write the ionic and net ionic equation for each
K2SO4(aq) + 2 AgNO3(aq) 2 KNO3(aq) + Ag2SO4(s)
2K+(aq) + SO42−(aq) + 2Ag+(aq) + 2NO3
−(aq)
2K+(aq) + 2NO3−(aq) + Ag2SO4(s)
2 Ag+(aq) + SO42−(aq) Ag2SO4(s)
Na2CO3(aq) + 2 HCl(aq) 2 NaCl(aq) + CO2(g) + H2O(l)
2Na+(aq) + CO32−(aq) + 2H+(aq) + 2Cl−(aq)
2Na+(aq) + 2Cl−(aq) + CO2(g) + H2O(l)
CO32−(aq) + 2 H+(aq) CO2(g) + H2O(l)
K2SO4(aq) + 2 AgNO3(aq) 2 KNO3(aq) + Ag2SO4(s)
Na2CO3(aq) + 2 HCl(aq) 2 NaCl(aq) + CO2(g) + H2O(l)
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Acid-Base Reactions
• Also called neutralization reactions because the acid and base neutralize each other’s properties
2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2 H2O(l)
• The net ionic equation for an acid-base reaction is
H+(aq) + OH(aq) H2O(l)as long as the salt that
forms is soluble in water
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Acids and Bases in Solution• Acids ionize in water to form H+ ions
more precisely, the H from the acid molecule is donated to a water molecule to form hydronium ion, H3O+
most chemists use H+ and H3O+ interchangeably
• Bases dissociate in water to form OH ionsbases, such as NH3, that do not contain OH ions, produce
OH by pulling H off water molecules• In the reaction of an acid with a base, the H+ from the
acid combines with the OH from the base to make water
• The cation from the base combines with the anion from the acid to make the salt
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HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
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Example: Write the molecular, ionic, and net-ionic equation for the reaction of aqueous nitric acid with aqueous calcium hydroxide
1. Write the formulas of the reactants
HNO3(aq) + Ca(OH)2(aq) 2. Determine the possible products
a) determine the ions present when each reactant dissociates or ionizes
(H+ + NO3−) + (Ca2+ + OH−)
b) exchange the ions, H+ combines with OH− to make H2O(l)
(H+ + NO3−) + (Ca2+ + OH−) (Ca2+ + NO3
−) + H2O(l)c write the formula of the salt
(H+ + NO3−) + (Ca2+ + OH−) Ca(NO3)2 + H2O(l)
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Example: Write the molecular, ionic, and net-ionic equation for the reaction of aqueous nitric acid with aqueous calcium hydroxide
3. Determine the solubility of the saltCa(NO3)2 is soluble
4. Write an (s) after the insoluble products and an (aq) after the soluble products
HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + H2O(l)
5. Balance the equation2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2 H2O(l)
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Example: Write the molecular, ionic, and net-ionic equation for the reaction of aqueous nitric acid with aqueous calcium hydroxide
6. Dissociate all aqueous strong electrolytes to get complete ionic equation
not H2O
2 H+(aq) + 2 NO3−(aq) + Ca2+(aq) + 2 OH−(aq)
Ca2+(aq) + 2 NO3−(aq) + H2O(l)
7. Eliminate spectator ions to get net-ionic equation
2 H+(aq) + 2 OH−(aq) H2O(l)
H+(aq) + OH−(aq) H2O(l)
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Practice – Predict the products and balance the equation
2 HCl(aq) + Ba(OH)2(aq) 2 H2O(l) + BaCl2(aq)
H2SO4(aq) + Sr(OH)2(aq) 2 H2O(l) + SrSO4(s)
(H+ + Cl−) + (Ba2+ + OH−) → (H+ + OH−) + (Ba2+ + Cl−)
HCl(aq) + Ba(OH)2(aq) → H2O(l) + BaCl2
(H+ + SO42−) + (Sr2+ + OH−) → (H+ + OH−) + (Sr2+ + SO4
2−) H2SO4(aq) + Sr(OH)2(aq) → H2O(l) + SrSO4
H2SO4(aq) + Sr(OH)2(aq) → 2 H2O(l) + SrSO4
HCl(aq) + Ba(OH)2(aq)
H2SO4(aq) + Sr(OH)2(aq)
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Titration• Often in the lab, a solution’s concentration
is determined by reacting it with another material and using stoichiometry – this process is called titration
• In the titration, the unknown solution is added to a known amount of another reactant until the reaction is just completed. At this point, called the endpoint, the reactants are in their stoichiometric ratio. the unknown solution is added slowly from an
instrument called a burettea long glass tube with precise volume markings that
allows small additions of solution
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Acid-Base Titrations• The difficulty is determining when there has been
just enough titrant added to complete the reaction the titrant is the solution in the burette
• In acid-base titrations, because both the reactant and product solutions are colorless, a chemical is added that changes color when the solution undergoes large changes in acidity/alkalinity the chemical is called an indicator
• At the endpoint of an acid-base titration, the number of moles of H+ equals the number of moles of OH also known as the equivalence point
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TitrationThe titrant is the base solution in the burette.
As the titrant is added tothe flask, the H+ reacts with the OH– to form water. But there is still excess acid present so the color does not change.
At the titration’s endpoint,just enough base has been added to neutralize all the acid. At this point the indicator changes color.
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Example 4.14:The titration of 10.00 mL of HCl solution of unknown concentration requires 12.54 mL of 0.100 M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution?
• Write down the given quantity and its units
Given: 10.00 mL HCl
12.54 mL of 0.100 M NaOH
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• Write down the quantity to find, and/or its units Find: concentration HCl, M
Example 4.14:The titration of 10.00 mL of HCl solution of unknown concentration requires 12.54 mL of 0.100 M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution?
InformationGiven: 10.00 mL HCl
12.54 mL of 0.100 M NaOH
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• Collect needed equations and conversion factors HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
1 mole HCl = 1 mole NaOH
0.100 M NaOH 0.100 mol NaOH 1 L sol’n
Example 4.14:The titration of 10.00 mL of HCl solution of unknown concentration requires 12.54 mL of 0.100 M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution?
InformationGiven: 10.00 mL HCl
12.54 mL of 0.100 M NaOHFind: M HCl
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• Write a conceptual plan
mLNaOH
LNaOH
molNaOH
molHCl
Information
Given: 10.00 mL HCl
12.54 mL of 0.100 M NaOH
Find: M HCl
Rel: 1 mol HCl = 1 mol NaOH
0.100 mol NaOH = 1 L
M = mol/L
Example 4.14:The titration of 10.00 mL of HCl solution of unknown concentration requires 12.54 mL of 0.100 M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution?
mLHCl
LHCl
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• Apply the conceptual plan
= 1.25 x 103 mol HCl
InformationGiven: 10.00 mL HCl
12.54 mL of 0.100 M NaOHFind: M HClRel: 1 mol HCl = 1 mol NaOH 0.100 mol NaOH = 1 L
M = mol/LCP: mL NaOH → L NaOH →
mol NaOH → mol HCl; mL HCl → L HCl & mol M
Example 4.14:The titration of 10.00 mL of HCl solution of unknown concentration requires 12.54 mL of 0.100 M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution?
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• Apply the conceptual plan
InformationGiven: 10.00 mL HCl
12.54 mL NaOHFind: M HClRel: 1 mol HCl = 1 mol NaOH 0.100 mol NaOH = 1 L
M = mol/LCP: mL NaOH → L NaOH →
mol NaOH → mol HCl; mL HCl → L HCl & mol M
Example 4.14:The titration of 10.00 mL of HCl solution of unknown concentration requires 12.54 mL of 0.100 M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution?
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Example 4.14:The titration of 10.00 mL of HCl solution of unknown concentration requires 12.54 mL of 0.100 M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution?
113Tro: Chemistry: A Molecular Approach, 2/e
InformationGiven: 10.00 mL HCl
12.54 mL NaOHFind: M HClRel: 1 mol HCl = 1 mol NaOH 0.100 mol NaOH = 1 L
M = mol/LCP: mL NaOH → L NaOH →
mol NaOH → mol HCl; mL HCl → L HCl & mol M
• Check the solutionHCl solution = 0.125 M
The units of the answer, M, are correct.The magnitude of the answer makes sense because
the neutralization takes less HCl solution thanNaOH solution, so the HCl should be more concentrated.
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Practice − What is the concentration of NaOH solution that requires 27.5 mL to
titrate 50.0 mL of 0.1015 M H2SO4?
H2SO4 + 2 NaOH Na2SO4 + 2 H2O50.0 mL 27.5 mL0.1015 M ? M
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1 mL= 0.001L, 1 LH2SO4 = 0.1015mol, 2mol NaOH : 1mol H2SO4
Practice — What is the concentration of NaOH solution that requires 27.5 mL to titrate 50.0 mL of 0.1015 M H2SO4?
2 NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2 H2O(aq)
the units are correct, the number makes because the volume of NaOH is about ½ the H2SO4, but the stoichiometry says you need
twice the moles of NaOH as H2SO4
50.0 mL of 0.1015 M H2SO4, 27.5 mL NaOH
M NaOH
Check:
Solution:
Conceptual Plan:
Relationships:
Given:
Find:
0.0500 L of 0.1015 M H2SO4, 0.0275 L NaOH
M NaOH
L H2SO4 mol H2SO4 mol NaOH
L NaOHM NaOH
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Gas-Evolving Reactions• Some reactions form a gas directly from the ion
exchange
K2S(aq) + H2SO4(aq) K2SO4(aq) + H2S(g)
• Other reactions form a gas by the decomposition of one of the ion exchange products into a gas and water
K2SO3(aq) + H2SO4(aq) K2SO4(aq) + H2SO3(aq)
H2SO3 H2O(l) + SO2(g)
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Electron Bookkeeping• For reactions that are not metal + nonmetal, or
do not involve O2, we need a method for determining how the electrons are transferred
• Chemists assign a number to each element in a reaction called an oxidation state that allows them to determine the electron flow in the reactioneven though they look like them, oxidation states
are not ion charges!oxidation states are imaginary charges assigned
based on a set of rules ion charges are real, measurable charges
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Rules for Assigning Oxidation States
• Rules are in order of priority1. free elements have an oxidation state = 0
Na = 0 and Cl2 = 0 in 2 Na(s) + Cl2(g)
2. monatomic ions have an oxidation state equal to their charge
Na = +1 and Cl = −1 in NaCl
3. (a) the sum of the oxidation states of all the atoms in a compound is 0
Na = +1 and Cl = −1 in NaCl, (+1) + (−1) = 0
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Rules for Assigning Oxidation States
3. (b) the sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion N = +5 and O = −2 in NO3
–, (+5) + 3(−2) = −1
4. (a) Group I metals have an oxidation state of +1 in all their compounds Na = +1 in NaCl
4. (b) Group II metals have an oxidation state of +2 in all their compounds Mg = +2 in MgCl2
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Rules for Assigning Oxidation States
5. in their compounds, nonmetals have oxidation states according to the table below
nonmetals higher on the table take priority
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Example: Determine the oxidation states of all the atoms in a propanoate ion, C3H5O2
–
• There are no free elements or free ions in propanoate, so the first rule that applies is Rule 3b
(C3) + (H5) + (O2) = −1
• Because all the atoms are nonmetals, the next rule we use is Rule 5, following the elements in order:H = +1O = −2
(C3) + 5(+1) + 2(−2) = −1
(C3) = −2
C = −⅔
Note: unlike charges, oxidation states can be fractions!
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Practice – Assign an oxidation state to each element in the following
• Br2
• K+
• LiF
• CO2
• SO42−
• Na2O2
Br = 0, (Rule 1)
K = +1, (Rule 2)
Li = +1, (Rule 4a) & F = −1, (Rule 5)
O = −2, (Rule 5) & C = +4, (Rule 3a)
O = −2, (Rule 5) & S = +6, (Rule 3b)
Na = +1, (Rule 4a) & O = −1 , (Rule 3a)
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Oxidation and ReductionAnother Definition
• Oxidation occurs when an atom’s oxidation state increases during a reaction
• Reduction occurs when an atom’s oxidation state decreases during a reaction
CH4 + 2 O2 → CO2 + 2 H2O−4 +1 0 +4 –2 +1 −2
oxidationreduction
123Tro: Chemistry: A Molecular Approach, 2/e
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Oxidation–Reduction• Oxidation and reduction must occur simultaneously
if an atom loses electrons another atom must take them
• The reactant that reduces an element in another reactant is called the reducing agent the reducing agent contains the element that is oxidized
• The reactant that oxidizes an element in another reactant is called the oxidizing agent the oxidizing agent contains the element that is reduced
2 Na(s) + Cl2(g) → 2 Na+Cl–(s)Na is oxidized, Cl is reduced
Na is the reducing agent, Cl2 is the oxidizing agent
124Tro: Chemistry: A Molecular Approach, 2/e
Copyright © 2011 Pearson Education, Inc.
Example: Assign oxidation states, determine the element oxidized and reduced,
and determine the oxidizing agent and reducing agent in the following reactions:
Fe + MnO4− + 4 H+ → Fe3+ + MnO2 + 2 H2O
0 −2+7 +1 +3 −2+4 +1 −2
OxidationReduction
Oxidizingagent
Reducingagent
125Tro: Chemistry: A Molecular Approach, 2/e
Copyright © 2011 Pearson Education, Inc.
Practice – Assign oxidation states, determine the element oxidized and reduced,
and determine the oxidizing agent and reducing agent in the following reactions:
Sn4+ + Ca → Sn2+ + Ca2+
F2 + S → SF4
+4 0 +2 +2Ca is oxidized, Sn4+ is reduced
Ca is the reducing agent, Sn4+ is the oxidizing agent
0 0 +4−1S is oxidized, F is reduced
S is the reducing agent, F2 is the oxidizing agent
126Tro: Chemistry: A Molecular Approach, 2/e