The Mathematics of Chemical Equations

Stoichiometry

Stoichiometry

• I. Stoichiometry- study of quantitative, or measurable, relationships that exist in chemical formulas and chemical reactions.

• II. Balanced Equations- The coefficients in a balanced chemical reaction indicate the number of moles of each substance participating in the chemical reaction.– Ex: 2H2 + O2 2H2O– 2 moles of hydrogen + 1 mole of oxygen yields 2 moles

of water.

Mole-Mole problems• Using a balanced equation you can determine amounts of

reactants or products.– Ex: Nitrous oxide is produced by the decomposition of

ammonium nitrate.• NH4(NO3) N2O + 2H2O• How many moles of N2O are produced from 2.25 moles of

NH4NO3?

Moles of given Moles of unknown

Use molar ratio to convert

Stoichiometry problems

• All stoichiometry problems are solved using a balanced chemical equation

• By doing so, you verify the Law of Conservation of Mass.

Mass-Mass Problems

• How many g of Cu(NO3)2 will be produced from 128 g of AgNO3?

• Cu + 2 AgNO3 Cu(NO3)2 + 2Ag

Mass of given

Moles of given Moles of unknown

Mass of unknown

Use molar mass Use molar

ratio

Use molar mass

Mass-mole problems

• How many moles of CO2 are produced from 64.8 g CO2?

• 2C2H2 + 5O2 4CO2 + 2 H2O

Mass of given Moles of given

Moles of unknown

convertUse molar ratio

Mole-mass problems

• What mass of H2O can be produced from 5.42 moles of glucose?

• C6H12O6 + 6 O2 4CO2 + 2 H2O

Moles of given

Moles of unknown

Mass of unknown

Use molar ratio

Use molar mass

Mass-Volume problems

• Sodium azide is the compound found in air bags. In a collision a sensor sets off a spark causing the NaN3 to decompose explosively, producing solid sodium and nitrogen gas: 2 NaN3 2Na + 3N2

• Assume an air bag contains 125 g of NaN3, what volume of N2 is produced?

• (Flow chart on next slide)

Mass-volume problems

Mass ofgiven

Moles ofgiven

Moles ofunknown

Volume of Unknown gas

Use molar mass

Use molar ratio

Use molar volume

Volume-volume problems

What volume of hydrogen gas is needed to react with 17.3 L of nitrogen gas?

N2 + 3H2 2NH3

Volume ofgiven

Moles ofgiven Moles of unknown

Volume of Unknown

Use molar volume

Use molar ratio

Use Molar volume

Limiting Reactants

• The limiting reactant (LR) is completely used up in the reaction.

• The other reactants will be left over, or “in excess”• The quantity (amount) of products formed in a rxn

are always determined by the quantity of the limiting reactant

• To determine the LR you do mass-mass problems

Limiting Reactant

• Determine the LR:

• Cu + 2AgNO3 Cu(NO3)2 + 2AgMass of

given

Moles ofgiven

Moles of unknown

Mass ofunknown

Determine LR

Percent Yield

• Ratio of the expected yield/actual yield

• Expected yield – amt of product that should be produced according to the balanced chemical equation.

• Actual yield- amt of product actually produced in the reaction

Percent yield

• % yield = (actual yield/expected yield) x 100

• Reasons for less than 100% yield– 1. Reactants don’t react– 2. Side reactions– 3. Loss of reactants/products– 4. Operator error