Stoichiometry

Module 6

Mole Relationships in Chemical Equations

• You do not always have stoichiometry problems where you are given the reactants and then asked for the product. You might be given the product and asked for the reactant.

Example Prob #1

• 2NaNO2 + O2 2NaNO3

• How many moles of oxygen gas must you use in order to make 15.0 moles of NaNO3? NaNO2?

Limiting Reactants and Excess Components

• Limiting reactant: the reactant in the equation that runs out first!

• This means that we only need to know the quantity of the limiting reactant because it will determine how much product can be formed in any reaction

Example Prob #2

• The first step in the process that converts iron ore into pure iron involves converting iron ore into iron oxide.

• 4FeS2 + 11O2 2Fe2O3 + 8SO2 Iron ore Iron Oxide

If an iron manufacturer starts with 1,256.0 moles of iron ore, how many moles of oxygen will be needed to react completely with the iron ore? How may moles of iron oxide will be made?

Relating Products to Reactants in Chemical Equations

• How do we know how much of a reactant we need to make a certain amount of product? We need to find out the minimum reactants needed to reach our desired product.

Example Prob #3

• Citric acid (C6H8O7), a component of fruit drinks, jams, and jellies, is produced using the following reaction:

• C12H22O11 + H2O + 3O2 2C6H8O7 + 4H2O

• If a fruit drink manufacturer decides she needs 1.4 x 105 moles of citric acid, what is the minimum reactants?

Limiting Reactant is Identified

• Stoichiometry can be used to determine the products when the limiting reactant is given!

Excess Reactant

Excess Reactant: when a chemist adds a lot of extra reactant to make sure that the other reactant is the limiting reactant

Example Prob #4

• Calcium chloride 6-hydrate (CaCl2H12O6) is a substance used to melt snow on streets and sidewalks. It is made from calcium carbonate (CaCO3) using the following reaction:

• CaCO3 + 2HCl + 5H2O CaCl2H12O6 + CO2

• If 3.49 moles of HCl are added to excess CaCO3

and excess water, how many moles of calcium chloride 6-hydrate will be made?

Stoichiometric Coefficients

• the number of molecules which participate in the reaction as written

•2H2 + O2 2H2O

Gay-Lussac’s LawThe stoichiometric coefficients in a chemical equation relate the volume of gases in equation as well as the number of moles of substances in the equation

REMEMBER: This law can only be used in problems that only have gases!!!

Example Prob #5

• A chemist decides to produce carbon dioxide gas using the following reaction:

• Na2CO3 (s) + 2HCl (g) 2NaCl (s) + CO2(g) + H2O (g)

• If the chemist wants to make 3.67 liters of CO2 gas, how many liters of HCl gas must be used?