Plan for Mon, 29 Sept 08• Exam I this Fri, 3 Oct 08

– Bring scantron form, scientific calculator, No. 2 pencils– Oct 3 is the add/change deadline

• Exp. 1 this Wed, 1 Oct – Buy Lab Manual– Make sure to complete pre-lab assignment before lab.– Don’t forget to bring your safety quiz!!

• Lecture– Using the mole to count particles (3.3)– Using molar mass as a conversion factor (3.4)– Chemical equations: balancing, interpreting (3.7-8)– Mole ratios and stoichiometry calculations (3.9)– Calculations involving a limiting reactant (3.10)

• Quiz 1 returned

If we had 1 mol of bicycles…– How many mol bike saddles

would we have?– Tires?– Spokes? (assuming 36

spokes/wheel)

If we had 1 mol of isopropyl alcohol (C3H8O)…

C

CH3

CH3

HO

H

– How many mol O atoms would we have?

– Carbon atoms?– Hydrogen atoms?

1 mol saddles

2 mol tires

72 mol spokes

1 mol O atoms

3 mol C atoms

8 mol H atoms

“Massive” Vocabularyatomic mass – average mass of an atom,

given in amumolecular mass – mass of a molecule,

given in amuformula mass – mass of the formula unit of

an ionic compound, given in amu. (e.g., NaCl, Ca2Br, BaSO4 are all formula units.)

molar mass – mass of one mole of a substance, given in g/mol

Examples: Finding # particles

1. How many water molecules are in a 10.0-g sample of water?

2. What is the total number of ions present in a 1.0-g sample of zinc oxide (ZnO)?

How many grams of oxygen corresponds to 0.50 moles?

? g O2 = 0.5 mol O2 32 g O2

1 mol O2

= 16 g O2

Molar Mass as a Conversion FactorMolar mass (the mass of one mole of a substance), allows us to convert between macroscopic and particulate points of view.

How many moles of oxygen are there in 1.2 grams?

? mol O2 = 1.2 g O2 1 mol O2

32 g O2

= 0.038 mol O2

Example: mass from moles

1. What is the mass of 7.65 x 1022 sulfur atoms?

2. Calculate the mass of 4.85 mol of acetic acid, CH3COOH.

Chemical Equations• Chemistry is the study of the rearrangement of matter

due to the flow of energy.• In a chemical reaction, some bonds are broken and

others are formed, resulting in a reorganization of the atoms.

• Atoms are neither created or destroyed in a chemical reaction!

+ +

When methane (CH4) reacts with oxygen, carbon dioxide and water are formed…

Reactants and products must occur in numbers that give the same number of each type of atom on both sides of the arrow.

Balancing Chemical Equations

1. Determine what reaction is occuring. What are the reactants, products, and physical states involved?

2. Write the unbalanced equation that summarizes the reaction in step 1.

3. Balance the equation by inspection. Do not change the identities (formulas) of any of the reactants or products.

Example

• Write balanced chemical equations for the following:

1. Aqueous solutions of lead(II) nitrate and sodium phosphate are mixed, resulting in the precipitate formation of lead(II) phosphate, with aqueous sodium nitrate as the other product.

2. The combustion of liquid ethanol (C2H5OH) forms carbon dioxide and water vapor. Combustion refers to the reaction of a substance with oxygen gas.

Interpreting Chemical Equations

• A reaction results in the rearrangement of atoms in one or more reactants to produce one or more products.

CO(g) + H2(g) CH3OH(l)

• What is this chemical equation telling us?

2

Interpreting (cont.)CO(g) + 2H2(g) CH3OH(l)

1 CO molecule + 2 H2 molecules 1 CH3OH molecule

1 dozen CO molecules

+2 dozen H2

molecules

1 dozen CH3OH molecules

6.022 x 1023 CO molecules

+2(6.022 x 1023) H2 molecules

6.022 x 1023 CH3OH molecules

1 mol CO molecules

+2 mol H2

molecules

1 mol CH3OH molecules

Stoichiometry• The relative quantities of reactants and products

involved in a chemical reaction.

1 ¾ cup cake flour3 tsp baking powder2 oz. baking chocolate1 ½ cup sugar ½ cup butter4 eggs½ cup milk

3 1/2 cup cake flour6 tsp baking powder4 oz. baking chocolate3 cup sugar 1 cup butter8 eggs1 cup milk

one chocolate cake

two chocolate cakes

Mole Ratios• We can use a balanced chemical equation to predict the

number of moles of products that a given number of moles of reactants will produce.

2H2O(l) 2H2(g) + O2(g)

2H2O(l) 2H2(g) + O2(g) 4H2O(l) 4H2(g) + 2O2(g)

Mole Ratios (cont)• What if we wanted to know the number of moles of H2 and O2

produced from the decomposition of 5.8 mol of H2O?

2H2O(l) 2H2(g) + O2(g)

• We know the following:

• We can represent these molar equivalencies as conversion factors:

2 mol H2O 2 mol H2

1 mol O22 mol H2O

2 mol H2O 2 mol H2~

2 mol H2O

2 mol H2

1 mol H2O

1 mol H2=

2 mol H2O 1 mol O2~

2 mol H2O

1 mol O2

Mole Ratios (cont)• Now we can answer the question…How many moles of H2

and O2 are produced from the decomposition of 5.8 mol of H2O?

2H2O(l) 2H2(g) + O2(g)

1 mol H2O

1 mol H25.8 mol H2O? mol H2 = 5.8 mol H2=

2 mol H2O

1 mol O25.8 mol H2O? mol O2 = 2.9 mol O2=

Example• Calculate the number of moles of oxygen

required to react exactly with 4.30 mol of propane, C3H8.

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

Stoichiometry and Mass• Recall: the number of moles of products

produced or reactants consumed represents numbers of particles, and we can’t count numbers of particles directly.

• In chemistry, we have to count by weighing.

• So, we need to use molar masses along with our mole ratios to convert between moles and grams.

Stoichiometry and Mass (cont)• Consider the reaction in which powdered aluminum

and finely ground iodine react violently to produce aluminum iodide:

Al(s) + I2(s) AlI3(s)

• Let’s say we have 35.0 g of aluminum powder.• How many grams of ground iodine do we need to

react completely with this amount of aluminum?

2 3 2

Stoichiometry and Mass (cont)• We need to construct the mole ratios from our

balanced chemical equation.2Al(s) + 3I2(s) 2AlI3(s)

• We want to do the following calculation:

2 mol Al 3 mol I2~2 mol Al

3 mol I2

mol Al present? g I2 required2 mol Al

3 mol I2=

But we have GRAMS of Al, not moles.

Stoichiometry and Mass (cont)• We need to convert our grams of Al into moles of Al:

mol Al present? g I2 required2 mol Al

3 mol I2=

1 mol Al

26.98 g Al35.0 g Al present

? g I2 required2 mol Al

3 mol I2= 1 mol Al

26.98 g Al35.0 g Al present

But we want GRAMS of I2, not moles.

Stoichiometry and Mass (cont)• We need to convert our moles of I2 into grams of I2:

1 mol I2

253.8 g I2mol I2 calculated

? g I2 required2 mol Al

3 mol I2= 1 mol Al

26.98 g Al35.0 g Al present

? g I2

2 mol Al

3 mol I2=1 mol Al

26.98 g Al35.0 g Al

1 mol I2

253.8 g I2495 g I2

=