StoichiometryStoichiometrySAVE PAPER AND INK!!! When you print out the notes on PowerPoint, print "Handouts" instead of "Slides" in the print setup. Also,

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Molar Mass of CompoundsMolar Mass of Compounds

The molar mass (MM) of a compound is The molar mass (MM) of a compound is determined the same way, except now you add determined the same way, except now you add up all the atomic masses for the molecule (or up all the atomic masses for the molecule (or compound)compound)Ex. Molar mass of CaClEx. Molar mass of CaCl22Avg. Atomic mass of Calcium = 40.08gAvg. Atomic mass of Calcium = 40.08gAvg. Atomic mass of Chlorine = 35.45gAvg. Atomic mass of Chlorine = 35.45gMolar Mass of calcium chloride = Molar Mass of calcium chloride =

40.08 g/mol Ca + (2 X 35.45) g/mol Cl40.08 g/mol Ca + (2 X 35.45) g/mol Cl 110.98 g/mol CaCl 110.98 g/mol CaCl22

20

Ca  40.08 17

Cl 35.45

FlowchartFlowchartAtoms or Molecules

Moles

Mass (grams)

Divide by 6.02 X 1023

Multiply by 6.02 X 1023

Multiply by atomic/molar mass from periodic table

Divide by atomic/molar mass from periodic table

The Mole is the central The Mole is the central conversion unitconversion unit

PracticePractice

Calculate the Molar Mass of Calculate the Molar Mass of calcium phosphatecalcium phosphate Formula = Formula = Masses elements:Masses elements:

Molar Mass = 3(40)+2(31)+8(16)= Molar Mass = 3(40)+2(31)+8(16)= 310 grams/mole310 grams/mole

Ca3(PO4)2

molar mass Avogadro’s numbermolar mass Avogadro’s number Grams Grams MolesMoles particles particles

Everything must go through Everything must go through Moles!!!Moles!!!

CalculationsCalculations

Chocolate Chip Cookies!!Chocolate Chip Cookies!!1 cup butter

1/2 cup white sugar

1 cup packed brown sugar

1 teaspoon vanilla extract

2 eggs

2 1/2 cups all-purpose flour

1 teaspoon baking soda

1 teaspoon salt

2 cups semisweet chocolate chips

Makes 3 dozen

How many eggs are needed to make 3 dozen cookies?

How much butter is needed for the amount of chocolate chips used?

How many eggs would we need to make 9 dozen cookies?

How much brown sugar would I need if I had 1 ½ cups white sugar?

Cookies and Chemistry…Huh!?!?Cookies and Chemistry…Huh!?!?

Just like chocolate chip Just like chocolate chip cookies have recipes, cookies have recipes, chemists have recipes as wellchemists have recipes as well

Instead of calling them Instead of calling them recipes, we call them reaction recipes, we call them reaction equationsequations

Furthermore, instead of using Furthermore, instead of using cups and teaspoons, we use cups and teaspoons, we use molesmoles

Lastly, instead of eggs, butter, Lastly, instead of eggs, butter, sugar, etc. we use chemical sugar, etc. we use chemical compounds as ingredientscompounds as ingredients

Chemistry RecipesChemistry Recipes

Looking at a reaction tells us how much of Looking at a reaction tells us how much of something you need to react with something you need to react with something else to get a product (like the something else to get a product (like the cookie recipe)cookie recipe)

Be sure you have a balanced reaction Be sure you have a balanced reaction before you start!before you start!

Example: 2 Na + ClExample: 2 Na + Cl2 2 2 NaCl 2 NaClThis reaction tells us that by mixing 2 moles of This reaction tells us that by mixing 2 moles of

sodium with 1 mole of chlorine we will get 2 moles sodium with 1 mole of chlorine we will get 2 moles of sodium chlorideof sodium chloride

What if we wanted 4 moles of NaCl? 10 moles? What if we wanted 4 moles of NaCl? 10 moles? 50 moles?50 moles?

PracticePractice

Write the balanced reaction for hydrogen gas Write the balanced reaction for hydrogen gas reacting with oxygen gas.reacting with oxygen gas.

2 H2 H22 + O + O22 2 H 2 H22OO How many moles of reactants are needed?How many moles of reactants are needed? What if we wanted 4 moles of water?What if we wanted 4 moles of water? What if we had 3 moles of oxygen, how much hydrogen What if we had 3 moles of oxygen, how much hydrogen

would we need to react and how much water would we would we need to react and how much water would we get?get?

What if we had 50 moles of hydrogen, how much oxygen What if we had 50 moles of hydrogen, how much oxygen would we need and how much water produced? would we need and how much water produced?

Mole RatiosMole Ratios Example: How many moles of chlorine is Example: How many moles of chlorine is

needed to react with 5 moles of sodium needed to react with 5 moles of sodium (without any sodium left over)?(without any sodium left over)?

2 Na + Cl2 Na + Cl22 2 NaCl 2 NaCl

5 mol mol?5 mol mol?Multiply the given by the Multiply the given by the unit you want to findunit you want to find, ,

divide by the divide by the unit you want to cancelunit you want to cancel. The plug in . The plug in the numbers that match the equality. the numbers that match the equality.

5 moles Na 1 mol Cl2

2 mol Na= 2.5 moles Cl2

3 Steps3 Steps

By using these 3 steps we can predict quantities of By using these 3 steps we can predict quantities of unknown products or reactants.unknown products or reactants.

Step 1 Step 2 Step 3Step 1 Step 2 Step 3

Moles known x Moles Unknown x G or L UnknownMoles known x Moles Unknown x G or L Unknown

Mole-Mass ConversionsMole-Mass Conversions

Most of the time in chemistry, the amounts are Most of the time in chemistry, the amounts are given in grams instead of molesgiven in grams instead of moles

We still go through moles and use the mole ratio, We still go through moles and use the mole ratio, but now we also use molar mass to get to gramsbut now we also use molar mass to get to grams

Example: How many grams of chlorine are required Example: How many grams of chlorine are required to react completely with 5.00 moles of sodium to to react completely with 5.00 moles of sodium to produce sodium chloride?produce sodium chloride?

2 Na + Cl2 Na + Cl22 2 NaCl 2 NaCl

5 mol g?5 mol g?

5.00 moles Na 1 mol Cl2 70.90g Cl2

2 mol Na 1 mol Cl2

= 177g Cl2

Mass-MoleMass-MoleWe can also start with mass and convert to We can also start with mass and convert to

moles of product or another reactantmoles of product or another reactantWe use molar mass and the mole ratio to get We use molar mass and the mole ratio to get

to moles of the compound of interestto moles of the compound of interestCalculate the number of moles of ethane (CCalculate the number of moles of ethane (C22HH66) )

needed to produce 10.0 g of waterneeded to produce 10.0 g of water 2 C2 C22HH66 + 7 O + 7 O22 4 CO 4 CO22 + 6 H + 6 H2200

mol? 10.0 gmol? 10.0 g10.0 g H2O 1 mol H2O 2 mol C2H6

18.0 g H2O 6 mol H20

= 0.185 mol C2H6

Mass-Mass ConversionsMass-Mass Conversions

Most often we are given a starting mass Most often we are given a starting mass and want to find out the mass of a product and want to find out the mass of a product we will get (called theoretical yield) or how we will get (called theoretical yield) or how much of another reactant we need to much of another reactant we need to completely react with it (no leftover completely react with it (no leftover ingredients!)ingredients!)

Now we must go from grams to moles, Now we must go from grams to moles, mole ratio, and back to grams of mole ratio, and back to grams of compound we are interested incompound we are interested in

Mass-Mass ConversionMass-Mass ConversionEx. Calculate how many grams of Ex. Calculate how many grams of

ammonia are produced when you react ammonia are produced when you react 2.00g of nitrogen with excess hydrogen.2.00g of nitrogen with excess hydrogen.

NN2 2 + 3 H+ 3 H2 2 2 NH 2 NH33

2.00 g g?2.00 g g?

2.00g N2 1 mol N2 2 mol NH3 17.06g NH3

28.02g N2 1 mol N2 1 mol NH3

= 2.4 g NH3

Volume to MassVolume to MassHow many grams of water are produced when How many grams of water are produced when

9.0 L of ethane reacts with unlimited oxygen? 9.0 L of ethane reacts with unlimited oxygen? (22.4 Liters gas / mole @ STP)(22.4 Liters gas / mole @ STP)

2 C2 C22HH66 + 7 O + 7 O22 4 CO 4 CO22 + 6 H + 6 H2200

Step 1Step 1 Step 2Step 2 Step 3Step 3

9L C9L C22HH44 x x 1 mol C1 mol C22HH66 x x 6 mol H6 mol H22OO xx 18 g H18 g H22O O

22.4L C22.4L C22HH6 6 2 mol C 2 mol C22HH6 6 1 mol H 1 mol H22OO

= 21.6 g H= 21.6 g H22O O

End of Part 1End of Part 1

Was it Was it HotHot oror

Not Not ??

Definitely HOT!Definitely HOT!

Limiting Reactant: CookiesLimiting Reactant: Cookies1 cup butter

1/2 cup white sugar

1 cup packed brown sugar

1 teaspoon vanilla extract

2 eggs

2 1/2 cups all-purpose flour

1 teaspoon baking soda

1 teaspoon salt

2 cups semisweet chocolate chips

Makes 3 dozen

If we had the specified amount of all ingredients listed, could we make 4 dozen cookies?

What if we had 6 eggs and twice as much of everything else, could we make 9 dozen cookies?

What if we only had one egg, could we make 3 dozen cookies?

Limiting ReactantLimiting Reactant

Most of the time in chemistry we have more of Most of the time in chemistry we have more of one reactant than we need to completely use one reactant than we need to completely use up other reactant.up other reactant.

That reactant is said to be in That reactant is said to be in excessexcess (there is (there is too much).too much).

The other reactant limits how much product we The other reactant limits how much product we get. Once it runs out, the reaction s. get. Once it runs out, the reaction s. This is called the This is called the limiting reactantlimiting reactant..

Limiting ReactantLimiting Reactant To find the correct answer, we have to try To find the correct answer, we have to try allall of of

the reactants. We have to calculate how much the reactants. We have to calculate how much of of aa product we can get from product we can get from eacheach of the of the reactants to determine which reactant is the reactants to determine which reactant is the limiting one.limiting one.

The The lowerlower amount of amount of aa product is the correct product is the correct answer.answer.

The reactant that makes the least amount of The reactant that makes the least amount of product is the product is the limiting reactantlimiting reactant. Once you . Once you determine the limiting reactant, you should determine the limiting reactant, you should ALWAYS start with it!ALWAYS start with it!

Be sure to pick Be sure to pick aa product! You can’t compare to product! You can’t compare to see which is greater and which is lower unless see which is greater and which is lower unless the product is the same!the product is the same!

Limiting Reactant: ExampleLimiting Reactant: Example 10.0g of aluminum reacts with 35.0 grams of 10.0g of aluminum reacts with 35.0 grams of

chlorine gas to produce aluminum chloride. Which chlorine gas to produce aluminum chloride. Which reactant is limiting, which is in excess, and how reactant is limiting, which is in excess, and how much product is produced?much product is produced?

2 Al + 3 Cl2 Al + 3 Cl22 2 AlCl 2 AlCl33 Start with Al:Start with Al:

Now ClNow Cl22::

10.0 g Al 1 mol Al 2 mol AlCl3 133.5 g AlCl3

27.0 g Al 2 mol Al 1 mol AlCl3

= 49.4g AlCl3

35.0g Cl2 1 mol Cl2 2 mol AlCl3 133.5 g AlCl3

71.0 g Cl2 3 mol Cl2 1 mol AlCl3

= 43.9g AlCl3

LimitingLimitingReactantReactant

LR Example ContinuedLR Example Continued

We get We get 49.4g49.4g of aluminum chloride from the given of aluminum chloride from the given amount of aluminum, but only amount of aluminum, but only 43.9g43.9g of aluminum of aluminum chloride from the given amount of chlorine. chloride from the given amount of chlorine. Therefore, chlorine is the limiting reactant. Once Therefore, chlorine is the limiting reactant. Once the 35.0g of chlorine is used up, the reaction the 35.0g of chlorine is used up, the reaction comes to a complete .comes to a complete .

Limiting Reactant PracticeLimiting Reactant Practice

15.0 g of potassium reacts with 15.0 g of 15.0 g of potassium reacts with 15.0 g of iodine. Calculate which reactant is limiting iodine. Calculate which reactant is limiting and how much product is made.and how much product is made.

2 K + I2 K + I22 2 KI 2 KI

15 g 15g limiting reactant?15 g 15g limiting reactant?

Finding the Amount of ExcessFinding the Amount of Excess

By calculating the amount of the excess By calculating the amount of the excess reactant needed to completely react with reactant needed to completely react with the limiting reactant, we can subtract that the limiting reactant, we can subtract that amount from the given amount to find the amount from the given amount to find the amount of excess.amount of excess.

Can we find the amount of excess Can we find the amount of excess potassium in the previous problem?potassium in the previous problem?

Finding Excess PracticeFinding Excess Practice 15.0 g of potassium reacts with 15.0 g of iodine. 15.0 g of potassium reacts with 15.0 g of iodine.

2 K + I2 K + I22 2 KI 2 KI We found that Iodine is the limiting reactant, and We found that Iodine is the limiting reactant, and

19.6 g of potassium iodide are produced.19.6 g of potassium iodide are produced.

15.0 g I2 1 mol I2 2 mol K 39.1 g K

254 g I2 1 mol I2 1 mol K= 4.62 g K USED!

15.0 g K – 4.62 g K = 10.38 g K EXCESS

Given amount of excess reactant

Amount of excess reactant actually used

Note that we started with the limiting reactant! Once you determine the LR, you should only start with it!

Limiting Reactant: RecapLimiting Reactant: Recap

1.1. You can recognize a limiting reactant problem because You can recognize a limiting reactant problem because there is MORE THAN ONE GIVEN AMOUNT.there is MORE THAN ONE GIVEN AMOUNT.

2.2. Convert ALL of the reactants to the SAME product (pick Convert ALL of the reactants to the SAME product (pick any product you choose.)any product you choose.)

3.3. The lowest answer is the correct answer.The lowest answer is the correct answer.4.4. The reactant that gave you the lowest answer is the The reactant that gave you the lowest answer is the

LIMITING REACTANT.LIMITING REACTANT.5.5. The other reactant(s) are in EXCESS.The other reactant(s) are in EXCESS.6.6. To find the amount of excess, subtract the amount used To find the amount of excess, subtract the amount used

from the given amount.from the given amount.7.7. If you have to find more than one product, be sure to If you have to find more than one product, be sure to

start with the limiting reactant. You don’t have to start with the limiting reactant. You don’t have to determine which is the LR over and over again!determine which is the LR over and over again!

Percent YieldPercent YieldWhen you want to calculate the amount of a When you want to calculate the amount of a substance that is produced compared to the substance that is produced compared to the amount you expected to produce:amount you expected to produce:

ActualActual x 100 = Percent Yield x 100 = Percent Yield

ExpectedExpected

If you produced 10 cookies but expected 12, your If you produced 10 cookies but expected 12, your yield is 10 ÷ 12 x 100 = 83% yieldyield is 10 ÷ 12 x 100 = 83% yield

The End of Stoichiometry The End of Stoichiometry For NowFor Now

Hot Hot

or or NotNot??

Ke$ha says definitely hot!Ke$ha says definitely hot!

The End The End