Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
112 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 11
11.3 Limiting Reactants
Rarely are the reactants in a chemical reaction present in the exact
mole ratios specified in the balanced equation. Usually, one or more
of the reactants are present in excess, and the reaction proceeds until
all of one reactant is used up. The reactant that is used up is called
the limiting reactant. The limiting reactant limits the reaction and,
thus, determines how much of the product forms. The left-over reactants are called excess reactants.
How can you determine which reactant in a chemical reaction is
limited? First, find the number of moles of each reactant by multiplying the given mass of each reactant by the inverse of the molar
mass. Next, determine whether the reactants are available in the
mole ratio specified in the balanced equation. A reactant that is
available in an amount smaller than that required by the mole ratio is
a limiting reactant.
After the limiting reactant has been determined, calculate the
amount of product that can ideally form from the given amount of
the limiting reactant. To do this, multiply the given number of moles
of the limiting reactant by the mole ratio that relates the limiting
reactant to the product. Then, convert moles of product to mass
using the molar mass of the product as the conversion factor.
Example Problem 11-5
Determining the Limiting Reactant
In the reaction below, 40.0 g of sodium hydroxide (NaOH) reacts
with 60.0 g of sulfuric acid (H 2 SO 4 ).
2NaOH(aq) ϩ H 2 SO 4 (aq) 0 Na 2 SO 4 ϩ 2H 2 O(g)
a. Which reactant is the limiting reactant?
b. What mass of Na 2 SO 4 can be produced using the given
quantities of the reactants?
a. To determine the limiting reactant, calculate the actual ratio of
available moles of reactants.
40.0 g NaOH ϫ
ϭ 1.00 mol NaOH
1 mol NaOH
ᎏᎏ
40.0 g NaOH