UNIT VII
Excess & Percentage Yield
Unit 7: Lesson 2
LIMITING REAGENTS & EXCESS AMOUNTS
SO far, we’ve only considered reactions assuming
that ALL of the reactants will be USED UP
entirely so zero is left at the end of the reaction.
Wellllll, this is not always true!
WHAT to do?!!?
VII.4 STOICHIOMETRY OF EXCESS
QUANTITIES



Often in a reaction, not all of the reactants are
used up – frequently one or more reactants are in
excess
LIMITING REAGENT: is the reactant you run
out of first; the smaller number!
EXCESS REAGENT: is the one you have left
over; the bigger number!
VII.4 STOICHIOMETRY OF EXCESS
QUANTITIES
Ex: you are given one dozen loaves of bread, a
gallon of mustard and three pieces of salami – how
many salami sandwiches can you make???


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Limiting reagent =
Excess reagents =
The limiting reagent determines how much
product you can make
VII.4 STOICHIOMETRY OF EXCESS
QUANTITIES
Well, how do we find out the limiting reagent
in a chemical reaction!?
In order to FIND OUT which reactant is
limiting…you do TWO stoichiometry problems:

The one that makes the least product is the
limiting reagent!!
VII.4 STOICHIOMETRY OF EXCESS QUANTITIES
Ex: #1
Copper reacts with sulfur to form copper (I)
sulfide. If 10.6g of copper reacts with 3.83g of S
how much product will be formed?
VII.4 STOICHIOMETRY OF EXCESS
QUANTITIES
Ex: #2
Mg(s) + 2HCl(g) → MgCl2(s) + H2(g)
If 10.1 mol of magnesium and 4.87 mol of HCl
gas are reacted, how many moles of gas will be
produced? How much excess reagent remains?
VII.4 STOICHIOMETRY OF EXCESS QUANTITIES
TRY:
If 10.3 g of aluminum are reacted with 51.7g of
CuSO4 how much copper will be produced? How
much excess reagent will remain?
VII.4 STOICHIOMETRY OF EXCESS QUANTITIES
Ex: #3
When 79.1 g of zinc are reacted with 1.05L of
2.00M HCl, to produce ZnCl2, and H2 gas, which
reactant will be in excess and by how much?
VII.5 PERCENTAGE YIELD
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
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Percentage Yield: is used to describe the
amount of product actually obtained as a
percentage of the expected amount.
2 reasons:
1. reactants may not all react
2. some products are lost
Yield: is the amount made in a chemical reaction
VII.5 PERCENTAGE YIELD:
THREE TYPES of YIELD
1. Actual Yield – what you get in the lab when the
chemicals are mixed
2.
Theoretical Yield – what the balanced equation
says you should make
3.
Percent Yield - Actual Yield (obtained)
100%
Theoretical Yield (expected)

Percent purity- pure x 100%
impure
x
VII.5 PERCENTAGE YIELD:

There are two possible situations:
1. Actual amount of PRODUCTS FOUND is
LOWER than the value expected.
To solve:
 calculate “amount of product expected” using
stoichiometry
 then multiply (x) “amount of product
expected” by the decimal equivalent of the %
yield or % purity
 Products multiply.

VII.5 PERCENTAGE YIELD:
2. Actual amount of REACTANTS NEEDED is
GREATER than the value expected

To solve:
 calculate the “amount of reactant needed”
using stoichiometry
 then divide (÷) “amount of reactant needed”
by the decmial equivalent of % yield or %
purity

Reactants divide
VII.5 PERCENTAGE YIELD:
Ex: #1
When 45.8g of potassium carbonate, K2CO3 are
reacted completely with an excess of HCl, 46.3 g
of KCl are produced. Water and CO2 also are
formed. Calculate the theoretical yield and the
percent yield of KCl.
VII.5 PERCENTAGE YIELD AND PERCENTAGE
PURITY:
Ex: #2
2Al + 3CuSO4 → Al2(SO4)3 + 3Cu
When 3.92 g of Al are reacted with excess
copper(II) sulfate, 6.78 g of copper is produced.
a) What is the actual yield?
VII.5 PERCENTAGE YIELD AND PERCENTAGE
PURITY:
Ex: #2 (continued)
2Al + 3CuSO4 → Al2(SO4)3 + 3Cu
When 3.92 g of Al are reacted with excess
copper(II) sulfate, 6.78 g of copper is produced.
b) What is the theoretical yield?
VII.5 PERCENTAGE YIELD AND PERCENTAGE
PURITY:
Ex: #2 (continued)
2Al + 3CuSO4 → Al2(SO4)3 + 3Cu
When 3.92 g of Al are reacted with excess
copper(II) sulfate, 6.78 g of copper is produced.
c) What is the percentage yield?
VII.5 PERCENTAGE YIELD AND PERCENTAGE
PURITY:
Ex: #2 (continued)
2Al + 3CuSO4 → Al2(SO4)3 + 3Cu
When 3.92 g of Al are reacted with excess
copper(II) sulfate, 6.78 g of copper is produced.
d) If you started with 9.73g of Al how much copper would
you expect?
VII.5 PERCENTAGE YIELD AND PERCENTAGE
PURITY:
Ex: #3
What mass of Fe2O3 is produced when 2.30 g of FeCO3 are
reacted with an excess of O2 according to the reaction:
4FeCO3 + O2 → 2Fe2O3 + 4CO2,
if the reaction has a 67.0 % yield?
VII.5 PERCENTAGE YIELD AND PERCENTAGE
PURITY:
Ex: #4
What mass of CO2 is required to make 8.00g of
K2CO3(s) according to the reaction:
4KO2(s) + 2CO2(g) → 2K2CO3(s) + 3O2(g)
if the reaction has a 85.0% yield?
HOMEWORK
Excess amounts: p.133 # 26-32
% questions: p. 137 # 33-37
UNIT 7 STOICHIOMETRY : FINITOOOOOOO!