Section 9-3
Limiting Reactants
and
Percent Yield
Example 1
You want to make smore’s according to the following recipe:
2 graham crackers + 1 marshmallow + 1 chocolate = 1 smore
If you have 17 graham crackers, 7 marshmallows and 9
chocolates, which is the limiting ingredient?
How many smore’s can you make?
Limiting Reactant (or Reagent)
The substance that is
completely used up first in a
reaction
The reactant that limits the
amounts of the other reactants
that can combine and the
amount of product that can
form in a chemical reaction.
Excess Reactant
• The substance that is
not used up
completely in a
reaction.
Sample Problem 9-6
Silicon dioxide (quartz) is usually quite unreactive, but reacts
readily with hydrogen fluoride according to the following
equation:
SiO2 (s) + 4 HF (g) → SiF4 (g) + 2 H2O (l)
If 2.0 mol of HF are exposed to 4.5 mol of SiO2, which is the
limiting reagent?
Practice Problem #1
Some rocket engines use a mixture of hydrazine, N2H4, and
hydrogen peroxide, H2O2, as the propellant. The reaction is
N2H4 (l) + 2 H2O2 (l) → N2 (g) + 4 H2O (g)
a. Which is the limiting reactant in this reaction when 0.750
mol of hydrazine is mixed with 5.00 mol of H2O2?
b. How much of the excess reactant, in moles, remains?
c. How much of each product, in moles, is formed?
Practice Problem #2
If 20.5 g of chlorine is reacted with 20.5 g of sodium, which
reactant is in excess? How do you know?
Sample Problem 9-7
The black oxide of iron, Fe3O4, is the mineral
magnetite. It can be made in the lab.
3 Fe (s) + 4 H2O (g) → Fe3O4 (s) + 4 H2 (g)
a. When 36.0 g of steam is mixed with 167 g of Fe,
which is the limiting reagent?
b. What mass of iron oxide is produced?
c. What mass of excess reagent remains when the
reaction is completed?
Practice Problem #1
Zinc and sulfur react to form zinc sulfide.
8 Zn (s) + S8 (s) → 8 ZnS (s)
a. If 2.00 mol of Zn are heated with 1.00 mol of S8, identify
the limiting reagent.
b. How many moles of excess reagent remain?
c. How many moles of the product are formed?
Practice Problem #2
Carbon reacts with steam to produce hydrogen and carbon monoxide.
a. If 2.40 mol of carbon are exposed to 3.10 mol of steam, identify the
limiting reagent.
b. How many moles of each product are formed?
c. What mass of each product is formed?
Theoretical Yield

The maximum amount of product that can be
produced from a given amount of reactant.
Actual Yield
 The
measured
amount of a product
obtained from a
reaction. In most
chemical reactions,
the amount of
product is less than
the theoretical
amount that you
could get.
Percent Yield
 The ratio of the actual
yield to the theoretical
yield, multiplied by 100.
actual
% yield = theoretical x 100
Sample Problem 9-8
Chlorobenzene is prepared by the following process:
C6H6 (l) + Cl2 (g) → C6H5Cl (s) + HCl (g)
When 36.8 g of C6H6 react with an excess of Cl2, the actual
yield of C6H5Cl is 38.8 g. What is the percent yield of
chlorobenzene?
Practice Problem #1
Methanol can be produced through the reaction of carbon
monoxide and hydrogen in the presence of a catalyst.
Pt
CO (g) + 2 H2 (g) → CH3OH (l)
If 75.0 g of CO reacts to produce 68.4 g of CH3OH, what is
the percent yield of CH3OH?
Practice Problem #2
Aluminum reacts with excess copper sulfate in the following
unbalanced equation:
Al (s) + CuSO4 (aq) → Al2(SO4)3 (aq) + Cu (s)
If 1.85 g of Al react and the percent yield of Cu is 56.6 %,
what mass of Cu is produced?
Assignment – Due Monday BOP
9-3 Textbook Problems
9-3 Worksheet
9 Element Handbook
Problems