Example of a mole to gram (2-step) stoichiometry problem
The Reaction: 6 CO2 + 6 H2O  1 C6H12O6 + 6 O2
• Question: If you have 5 moles of H2O how many grams of
oxygen can you produce?
• Conversion: 5 moles H2O to grams of O2
Start with the
amount given in the
question.
Convert from moles
to grams. Multiply
by molar mass of O2
(16x2 = 32 g O2/mol)
5 mol H2O X 6 mol O2 = 5 mol O2 X 32 g O2 =
6 mol H2O
1 mol O2
Multiply by the mole ratio from
the balanced equation with the
unknown in the numerator. This
cancels the given moles.
160 g O2
This is the answer.
Notes: What is percent yield?
• Percent yield describes the efficiency of a chemical reaction.
• It is a comparison between the actual yield and the
theoretical yield.
• Calculate the percent yield:
actual yield
theoretical yield
X 100% = percent yield
•• Calculate
percent
yield:
Calculatethe
the
percent
yield:
actual
= percent
yield yield
actualyield
yield X 100%
X 100%
= percent
theoretical
theoreticalyield
yield
You
try 1: The
Thetheoretical
theoreticalyield
yield
200when
g. The
yield
150 g.and
Example:
of is
silver
youactual
combine
8g iscopper
What
is the ispercent
yield?
1.7g AgNO
1.08 g Ag.
The actual yield is 0.92 g Ag. What is the
3
percent yield?
You try 2: The theoretical yield is 50 g. The actual yield is 25 g.
WhatAgis theXpercent
0.92g
100%yield?
= 85%
1.08 g Ag
You try 3: When 34 g ammonia are combined with excess oxygen,
the theoretical yield of nitrogen gas is 28 g. The actual yield is
22 g. What is the percent yield of nitrogen gas?
Notes: Percent Yield (to practice – p. 319 #6, p. 331 #34)
• Example: hydrogen and oxygen form water
2 H2 (g) + O2 (g) → 2 H2O (l)
• Identify the limiting reactant and the theoretical yield of water (grams)
when 10 g hydrogen react with 32 g oxygen. The actual yield is 27g water.
• What is the percent yield?
1: Split
into
two
stoichiometry
problems.
a)Step
How
manythe
g Hquestion
O
form
from
10
g
H
?
2
2
Step 2: Solve both problems.
Step 3: Compare answers. The smaller product amount is the
theoretical yield. The limiting reactant is the reactant that
b) How
many g less
H2Oproduct.
form from 32 g O2?
that formed
Step 4: Percent yield – divide actual yield/theoretical x 100%
Notes: Percent Yield (to practice – p. 319 #6, p. 331 #34)
• Example: reaction of ammonia and oxygen
4 NH3 + 3 O2 → 6 H2O + 2 N2
• Identify the limiting reactant and the theoretical yield of Nitrogen (grams
of N2) when 34 g ammonia (NH3 ) react with 64 g oxygen (O2).
• The actual yield is 24 g N2. What is the percent yield?
Step
1: Split
the
question
intofrom
two34stoichiometry
problems.
a) How
many
grams
of N2 form
g NH3 ?
Step 2: Solve both problems.
Step 3: Compare answers. The smaller product amount is the
theoretical yield. The limiting reactant is the reactant that
b)that
How
many grams
of N2 form from 64 g O2?
formed
less product.
Step 4: Percent yield – divide actual yield/theoretical x 100%
N2 + 3 H2 2 NH3
1. What is the mole ratio of H2 to NH3?
3 mol H2
2 mol NH3
2. How many moles of H2 are needed to produce 10 moles NH3?
10 mol NH3
X
3 mol H2 =
2 mol NH3
15 mol H2
3. How many moles H2 are needed to produce 170 grams NH3? (molar mass
of NH3 is 17g/mol)
170g NH3 ÷ 17 g/mol = 10 mol NH3 X 3 mol H2
2 mol NH3
= 15 mol H2
4. How many grams H2 are needed to produce 170 grams NH3? (NH3
=17g/mol; H2 = 2g/mol)
170g NH3 ÷ 17 g/mol = 10 mol NH3 X
3 mol H2
=15 mol H2X 2 g/mol H2 = 30 g H2
2 mol NH3
Notes: Limiting Reactant & Product Yield
• Limiting reactant = the substance that controls the quantity of
product that can form in a chemical reaction.
– The limiting reactant runs out first.
• Excess reactant = the substance that is not used up
completely in a reaction.
• Product Yield = the amount of product formed
• Theoretical Yield = amount of product predicted from
calculations.
Think, Pair, Share:
• What was the limiting reactant in your group’s
s’more experiment?
• What were the excess reactants?
Notes: Limiting Reactant & Product Yield
• Example 3: Copper and Silver Nitrate
Cu (s) + 2AgNO3 (aq) → 2Ag (s) + Cu(NO3)2 (aq)
• Identify the limiting reactant and the theoretical (calculated) yield
of silver (Ag) in grams, when 128 g Cu reacts with 170 g AgNO3 .
• Step 1: Separate the question into two stoichiometry problems:
• How many grams Ag will be produced from 128 g Cu?
• How many grams Ag will be produced from 170 g AgNO3?
• Step 2: Solve both problems.
• Step 3: Compare your answers. The smaller answer has the limiting
reactant and is the theoretical yield.
Notes: Limiting Reactant & Product Yield
• Example 3: Copper and Silver Nitrate
Cu (s) + 2AgNO3 (aq) → 2Ag (s) + Cu(NO3)2 (aq)
• Identify the limiting reactant and the theoretical (calculated) yield
of silver (Ag) in grams, when 128 g Cu reacts with 170 g AgNO3 .
• How many grams Ag will be produced from 128 g Cu?
• How many grams Ag will be produced from 170 g AgNO3?
Practice: Limiting Reactant & Product Yield
• Show all your work on your own sheet of paper.
• Pre-Lab Problems: Copper and Silver Nitrate
Cu (s) + 2AgNO3 (aq) → 2Ag (s) + Cu(NO3)2 (aq)
1. Identify the limiting reactant and the theoretical (calculated) yield
of silver (Ag) in grams, when 64 g Cu reacts with 170 g AgNO3 .
2. Identify the limiting reactant and the theoretical (calculated) yield
of silver (Ag) in grams, when 6.4 g Cu reacts with 8.5 g AgNO3 .
3. Identify the limiting reactant and the theoretical (calculated) yield
of silver (Ag) in grams, when 32 g Cu reacts with 340 g AgNO3 .
•
•
•
Step 1: Separate the question into two stoichiometry problems:
Step 2: Solve both problems.
Step 3: Compare your answers. The smaller answer has the limiting reactant and is
the theoretical yield.
Check Your Understanding: 3e
• Copper and Silver Nitrate Single Displacement Reaction:
Cu (s) + 2AgNO3 (aq) → 2Ag (s) + Cu(NO3)2 (aq)
1. What is the mole ratio of silver (Ag) to copper (Cu) in the balanced
equation?
2. What is the mole ratio of silver (Ag) to silver nitrate (AgNO3) in the
balanced equation?
3. How many grams Ag will be produced from 640 g Cu?
4. How many grams Ag will be produced from 1700 g AgNO3?
5. Identify the limiting reactant and the theoretical (calculated) yield
of silver (Ag) in grams, when 640 g Cu reacts with 1700 g AgNO3 .