Stoichiometry

The Math of Chemical Reactions

Unit 9

Stoichiometry

The study of the mathematical relationships between the substances in balanced chemical reactions

Allows for calculation of quantities of reactants or products in a chemical reaction using the relationships found in the balanced chemical equation

Stoichiometry (cont.)

Allows for calculation of quantities of reactants or products in a chemical reaction

Example: According to the following reaction, how many grams of water can you make if you start with 10.0 grams of hydrogen gas?

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

Mole Ratios

Recall: Coefficients in a balanced chemical equation indicate the relative amounts of each substance involved in the reaction

In the following reaction: N2O3 + H2O 2 HNO3

One way to describe the ratios involved in the reaction would be “one molecule of dinitrogen trioxide plus one molecule of water yields two molecules of nitrous acid”

Mole Ratios

However, since these are only ratios, using the same equation, N2O3 + H2O 2 HNO3

It is equally true that, “one mole of dinitrogen trioxide plus one mole of water yields two moles of nitrous acid” These mole relationships are called “mole

ratios”

Mole Ratios

Example #1: What is the mole to mole ratio of oxygen to water in the following equation:

2 H2 + O2 2 H2O

Mole Ratios

Example #2: What is the mole ratio of sodium hydroxide to sodium sulfate in the following equation:

H2SO4 + 2 NaOH Na2SO4 + 2 H2O

Stoichiometric Calculations

Mole to Mole Calculations – converting from moles of one compound to moles of a different compound 1) Write and balance the equation if not done 2) The coefficients in front of the mentioned

compounds serve as the conversion factor (mole ratio) between the two desired compounds

3) Use dimensional analysis & the mole ratio to convert from moles of compound #1 to moles of compound #2

Mole to Mole Calculations

Ex #1: In the equation below, if 0.5 moles of magnesium hydroxide react, how many moles of water would be produced?

2 H3PO4 + 3 Mg(OH)2 Mg3(PO4)2 + 6 H2O

Mole to Mole Calculations

Ex #2: In the equation below, if 0.5 moles of H3PO4 react, how many moles of water would be produced?

2 H3PO4 + 3 Mg(OH)2 Mg3(PO4)2 + 6 H2O

Mass to Mass Calculations 1) Convert from grams of given compound to

moles of known compound (using molar mass) 2) Convert from moles of given to moles of

unknown (using mole ratio) 3) Convert from moles of unknown to grams of

unknown (using molar mass)

Grams of Given

Moles of Given

Moles of

Unknown

Grams of

Unknown

Use molar mass

of given

Use molar

mass of unknown

Use mole ratio

Mass to Mass Calculations

Master Formula for Mass to Mass Calculations

unknown of mol 1

(g) unknown mass Molar

(mol) given of tCoefficien

(mol) unknown of tCoefficien

(g) given mass Molar

given of mol 1(g) Given

Mass to Mass Calculations

Ex #1: In the balanced equation below, how many grams of carbon dioxide will be produced by the reaction of 108 grams of C5H12?

C5H12 + 8 O2 5 CO2 + 6 H2O

Mass to Mass Calculations

Ex #2: In the balanced equation below, how many grams of carbon dioxide will be needed to completely react with 11.5 grams of oxygen?

C5H12 + 8 O2 5 CO2 + 6 H2O

Percent Yield

Theoretical yield – amount of product that should be formed when the limiting reacting (the reactant that runs out first) is completely consumed

Actual yield – the actual amount of product produced in a reaction

Percent yield – 100% yieldltheoretica

yieldactual

Percent Yield

Example #1: If a student made 1.72 g of NaCl in the lab when they should have theoretically made 2.00 g, what is the percent yield?

100% yieldltheoretica

yieldactual

Percent Yield

Example #2: If a student made 4.5 g of H2SO4 when they should have made 5.0 grams, what is the percent yield of the reaction? Does this percent yield make sense?

100% yieldltheoretica

yieldactual