Stoichiometry Notes

ChemistryMrs. Stoops

Stoichiometry NotesConverts grams of one substance into a different

substance.Uses:• Balanced chemical equations• Mole ratio• Mole conversions (grams moles)

Recipe ConnectionChocolate chip cookies – makes 60 cookies2 1/4 cups all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 1 cup (2 sticks) butter, softened 3/4 cup granulated sugar 3/4 cup packed brown sugar 1 teaspoon vanilla extract 2 large eggs 2 cups (12-oz. pkg.)

NESTLÉ® TOLL HOUSE® Semi-Sweet Chocolate Morsels

Mole ratioEX: SiO2 + 3 C SiC + 2 CO

14 g C ? g COThe coefficients represent the ratios of all the

reactants and products to each other1 mole SiO2 = 3 mole C = 1 mole SiC = 2 mole CO

SO we can take the parts we want out to solve our problem.

MOLE RATIO 3 mole C = 2 mole CO

The processEX: SiO2 + 3 C SiC + 2 CO

14 g C ? g CO

You can change the mole C into mole of CO using mole ratio, then . . .

If you can change the 14 g C into mole C using the molar mass, then . . .

You can change the mole CO into gram CO using molar mass!

Solve the problemEX: SiO2 + 3 C SiC + 2 CO

14 g C ? g CO

14gCx x12gC1molC x3molC

2molCO1molCO

28gCOThe set-up:

21.8gCO

Example #2EX: 2 C2H2 + 5 O2 4 CO2 + 2H2O

23 g CO2 ? gH2O

x23gCO2 x44gCO1molCO

2

2 x4molCOO2molH2

2 O1molHO18gH2

2

O4.7gH2

Part 2A + B C 10 g A ? g C

10 g A x 1 mol A x 1 mol C x ~ g C = g C ~ g A 1 mol A 1 mol C

gA mol A mol C g C molar mole molar mass ratio mass

The basic set-up

ROAD MAP

Other versions #1EX: 2H2 + O2 2H2O

2.1 mol O2

? gH2O

The set-up:

Moles A to grams C

2 fractions

x2.1molO2 x1molOO2molH2

2 O1molHO18gH2

2 O75.6gH2

MoleRatio

MolarMass

Other versions #2EX: 2H2 + O2 2H2O

3.1 mol H2O

? mol H2

The set-up:

Moles A to moles C

1 fraction

Ox3.1molH2 O2molH2molH

2

223.1molH

MoleRatio

You have the following materials:19 slices of bread10 slices of cheese12 slices of lunch meat10 slices of tomato

Each sandwich needs:2 slices of bread1 slices of cheese1 slices of lunch meat1 slices of tomato

What runs out first?

How many sandwiches do you make?

Limiting Reactant Pt3Limiting Reactant – “Which runs out first?”

EX: Ca(OH)2 + 2HCl CaCl2 + 2H2OIf you have 6.3 mol HCl and 2.8 mol Ca(OH)2 which will be completely used up?

2 reactants; pick one!Start with one number given in the problem

And change it to the other number given

Moles to moles 1 fraction

ContinuedxH)2.8molCa(O 2

21molCa(OH)2molHCl 5.6molHCl

6.3molHClx 2molHCl1molCa(OH)2 2H)3.2molCa(O

OR NEEDED

Which limits?

HAVE

Ca(OH)2 Limits

Limiting exampleEX: 2 KBr + Cl2 2KCl + Br2

3.205 g KBr

0.855 g Cl2

2

221molCl70gCl

2molKBr1molCl

KBr g 119KBr mol 13.205gKBr

OR

1molKBr119gKBr

1molCl2molKBr

70gCl1molCl0.855gCl

22

22

Limiting Reactant = Cl2

0.943 g Cl2

2.907 g KBr

Which limits?

A little twist . . . The question may ask you how much you actually produce.

First you find the limiting reactant, then . . .Take the limiting reactant and do stoichiometry on it to find the product they ask for.

If you follow the recipe from the sandwiches and you have enough ingredients to make 15 sandwiches.

BUT, the tomato is bad in spots, so only make 11 sandwiches.

What is your percent yield?

100made have should yousandwiches 15made actually sandwiches 11

73 %

Percent Yield Pt 4

Percent Yield = How much we made compared to how much we could have made.

100 yieldlTheoretica yieldActual yieldpercent

In real words . . .

100made have should youwhatfound youwhat yieldpercent

Will NEVER be more than 100%If you were perfect!

Always start with the reactant

EX: C6H6 + Cl2 C6H5Cl + HClWhen 36.8 g C6H6 reacts, the actual yield is 38.8 g C6H5Cl. What is the percent yield?

ClHC 1molClHC 112g

HC 1molClHC 1mol

HC 78gHC 1molH36.8gC

56

56

66

56

66

6666

52.8 g C6H5Cl

100ClHC 52.8gClHC 38.8g

56

56 73.4 %