StoichiometryLimiting

Reactants

StoichiometryStoichiometry enables us to compare amounts of two substances in a balanced chemical reaction.

Simple ExampleIf you have 6 slices of bread, how many ham sandwiches could you make?

2 Bread + 1 Ham → 1 Sandwich

6

2

1 3

Mole RatiosA mole ratio is a whole number relationship between two substances in a balanced chemical equation.

Converting Substances2Na(s) + Cl2(g) → 2 NaCl(s)

How many moles of salt could be made if you had 5.00 moles of chlorine gas?ANALYZE (have/need, conv.

factor)PLAN (setup dimensional analysis)

EVALUATE (crunch numbers)

5.00 mol Cl2 1 mol Cl22 mol NaCl 10 mol NaCl

Converting Mass to Mass2H2(g) + O2(g) → 2H2O(l)

How many grams of water could be made if you had 75.0 grams of oxygen?ANALYZE (have/need, conv.

factors)

PLAN (setup dimensional analysis)

EVALUATE (crunch numbers)

g O2 mol O2 mol H2O g H2O 75.0g O2 32.00g O2

1mol O2 1mol O2

2 mol H2O 1 mol H2O18.02g H2O 84.5g H2O

Limiting ReactantsThe first species that runs

out in a reaction prevents the reaction from continuing. It is the limiting reactant.

1 body + 4 tires → 1 car

Excess ReactantsThe species that are left

over from the beginning are excess reactants.

1 body + 4 tires → 1 car8 bodies 1 body4 tires 32 tires48 tires – 32 tires = 16 excess tires

Percentage YieldWe use percentage yield to

determine how efficient a chemical reaction is. x 100This can be done with grams or moles.

Reactions cannot exceed 100% yield.

Percentage Yield ExampleIf 152 grams of CO2 are

produced in a reaction, but the theoretical yield was calculated to be 175 grams of CO2, what is the percentage yield?

Percentage Yield PracticeFind the percentage yield if

5.50 moles of NaCl are produced from the reaction of 6.00 moles of sodium with excess chlorine gas.__Na + __Cl2 → __NaCl

Percent CompositionThe percent composition of

a substance indicates what percent of each atom is present in the compound (by mass). x 100The sum of the percent compositions must be 100%

Percent Composition

ExampleFind the percent composition of carbon in a sample of CO2.