Calculating Limiting and
Excess Reagents
Section 5.5
SCH 3U
The Mole Ratio
 The quantities of substances that react in a chemical
reaction are determined by the mole ratio given by the
balanced chemical equation
 but, in the real world, it is less likely for the reactants to be
in the mole ratios given by the equation – WHY??
The Answer
 In nature, compounds are found in random quantities
 In labs / industry, we might want to ensure that one
reactant is completely used up
The Excess Reagent
 In any chemical reaction, one reactant is always
present in EXCESS - in other words:
 the reagent that is not completely used up in the reaction
 there will be some left over at the end of the reaction
 in order to provide a reagent in excess, chemists add
10% to the mass required
The Limiting Reagent
 The other reactant is LIMITING - in other words:
 it is completely used up in the reaction
 the reagent in a chemical reaction that limits the amount
of product that is able to form
Kitchen Stoichiometry – an
analogy
 A recipe to make 30 candies requires the following:
 2 cups water
 5 cups sugar
 1 cup strawberries
 Equation
 2 c water + 5 c sugar + 1 c strawberries  30 candies
Questions:
a. If you were given 10 cups of water, 10 cups of sugar and
10 cups of strawberries, which ingredient would limit the
amount of candies being made? Which reagent is in
excess?
Questions:
b. How many candies can you make with the ingredients
given in part a?
Questions:
c. How much of the excess reagents will be left over?
Example 1:
 Lithium nitride reacts with water to produce ammonia gas and
lithium hydroxide. If 4.5 g of lithium nitride and 5.8 g of water
is available:
a. How much ammonia gas (in grams) can be produced?
 Step 1: Write a balanced chemical equation.
 Step 2: Choose a product – pick the one that you will need to
calculate the amount of (ammonia).
 Step 3: Determine the amount of ammonia that can be
produced from each of the reactants.
 Step 4: Compare the amounts of product formed from the
lithium nitride and from the water – the one that produces the
least ammonia is the limiting reagent
Example 1:
 Lithium nitride reacts with water to produce ammonia gas and
lithium hydroxide. If 4.5 g of lithium nitride and 5.8 g of water
is available:
b. How much (in grams) of the excess reagent will be left over?
Li3N(s)
+
3H2O(l) →
3LiOH(aq) +
NH3(g)
Example 2:
 Nitrogen and hydrogen react to form ammonia according to
the following equation:
N2(aq)
+
H2(g) →
NH3(g)
If 28.02 g of N2 is to be completely used up, what is a
reasonable mass of H2 to use in the reaction?