Agenda: 4/21
Homework: Complete
practice problems
Objective: To determine quantities needed for chemical
reactions in research or manufacturing: Stochiometry
Review: Set up of Stoichiometry Problems
Graphic Organizer for Stoichiometry
Class Practice: Mixed Problems
Percent Yield – Practice Problems
Mixed Stoichiometry Practice Problems - Pairs
1
Essential to Stoichiometry
Set-up of problems
Given:
Amount &
Unit
Do I need to
convert the
given to
moles?
Mole Ratio
Do I need to
convert from
moles to
another unit?
Unknown:
Unit
The mole ratio is the heart of
the stoichiometry calculation
2
Mole Ratio
Mole ratio: Mole Unknown
Mole Given
Mole ratio: Used the coefficients of a
balanced chemical
equation
3
Graphic organizer
Volume of a
Gas
Mole
Particles
Volume of
a Gas
Mass
X
Mass
Mole Ratio
Mole________
Mole
Mole
Particle
s
4
Putting all the steps together in a
stoichiometry calculation
Given:
Amount &
Unit
Do I need to
convert the
given to
moles?
Mole Ratio
Do I need to
convert from
moles to
another unit?
Unknown:
Unit
5
Putting all the steps together in a
stoichiometry calculation
• Propane (C₃H₈) reacts with oxygen to
produce carbon dioxide and water
vapor.
• Balanced equation:
6
Example 1:
• How many moles of oxygen are required to
completely react 10 moles of propane?
Given:
Amount &
Unit
Do I need to
convert the
given to
moles?
Mole Ratio
Do I need to
convert from
moles to
another unit?
Unknown:
Unit
7
Example 1:
• How many moles of oxygen are required to
completely react 10 moles of propane?
Given:
Amount &
Unit
Do I need to
convert the
given to
moles?
Mole Ratio
Do I need to
convert from
moles to
another unit?
Unknown:
Unit
8
Example 2:
• How many grams of oxygen are required to
completely react 10 moles of propane?
Given:
Amount &
Unit
Do I need to
convert the
given to
moles?
Mole Ratio
Do I need to
convert from
moles to
another unit?
Unknown:
Unit
Molar mass of oxygen:
9
Example 3:
• How many liters of oxygen are required to
completely react 10 moles of propane?
Given:
Amount &
Unit
Do I need to
convert the
given to
moles?
Mole Ratio
Do I need to
convert from
moles to
another unit?
Unknown:
Unit
1 mole of oxygen at STP = ____________ liters
10
Magnesium reacts with hydrochloric acid
to yield magnesium chloride and
hydrogen gas.
• Balanced equation:
11
Example 4:
• How many moles of magnesium chloride will be produced if a
chemist starts with 120 grams of magnesium metal and
plenty* of hydrochloric acid?
*Chemistry language: “with an excess of hydrochloric acid”
Given:
Amount &
Unit
Do I need to
convert the
given to
moles?
Mole Ratio
Do I need to
convert from
moles to
another unit?
Unknown:
Unit
Molar mass of magnesium:
12
Example 5:
• How many grams of magnesium chloride will be produced if a
chemist starts with 60 grams of magnesium metal and an
excess of hydrochloric acid?
Given:
Amount &
Unit
Do I need to
convert the
given to
moles?
Molar mass of magnesium:
Mole Ratio
Do I need to
convert from
moles to
another unit?
Unknown:
Unit
Molar mass of magnesium chloride:
13
Example 6:
• How many liters of Hydrogen gas will be produced
with 60 grams of hydrochloric acid and an excess of
magnesium?
Given:
Amount &
Unit
Do I need to
convert the
given to
moles?
Mole Ratio
Molar mass of hydrochloric acid:
Do I need to
convert from
moles to
another unit?
Unknown:
Unit
1 mole of hydrogen gas at STP:
14
Percent Yield
• Stoichiometry calculations always give the
maximum amount that can be produced.
This is called the _________ ________.
• In real life, the maximum amount is never
produced. Reactions do not proceed
perfectly. The _________ ________
is produced.
15
Percent Yield
• Percent yield:
Actual Yield x 100 = %
Theoretical Yield
• In the last stoichiometry problem,
661.5 grams of sulfuric acid was calculated.
However, the sample was weighed and found to
be 650.7 grams. The percent yield is:
______________ x 100 =
%
Note: the % yield is always less than 100%
16
Percent Yield Calculations
1)Balance this equation and state which of the six types of
reaction is taking place:
____ Mg + ____ HNO3  ____ Mg(NO3)2 + ____ H2
Type of reaction: __________________________
2)If I start this reaction with 40 grams of magnesium and an
excess of nitric acid, how many grams of hydrogen gas will I
produce?
3)If 1.7 grams of hydrogen is actually produced, what was my
percent yield of hydrogen?
17
4)Balance this equation and state what type of reaction is taking place:
____ NaHCO3  ____ NaOH + ____ CO2
Type of reaction: __________________________
5) If 25 grams of carbon dioxide gas is produced in this reaction, how
many grams of sodium hydroxide should be produced?
6) If 50 grams of sodium hydroxide are actually produced, what was
my percent yield?
18
Check answers
1) Balance this equation and state which of the six types of reaction is taking place:
1 Mg + 2 HNO3  1 Mg(NO3)2 + 1 H2
Type of reaction: single displacement
2) If I start this reaction with 40 grams of magnesium and an excess of nitric acid, how many
grams of hydrogen gas will I produce?
3.3 grams (when Mg atomic mass = 24.3 grams)
3) If 1.7 grams of hydrogen is actually produced, what was my percent yield of hydrogen?
52%
4)Balance this equation and state what type of reaction is taking place:
1 NaHCO3  1 NaOH + 1 CO2
Type of reaction: decomposition
5) If 25 grams of carbon dioxide gas is produced in this reaction, how many grams of sodium
hydroxide should be produced?
22.7 grams NaOH
6)If 50 grams of sodium hydroxide are actually produced, what was my percent yield?
50/22.7 x 100% = 220%
Hopefully, you understand that this is not a reasonable answer to this question and
indicates that something very wrong happened during this reaction.
19
Manufacturing Sulfuric Acid
Using stoichiometry in an industrial
application
20
Here is a typical problem:
•
Sulfuric acid, an important chemical in
industry, is manufactured and sold by XYZ
company.
• Sulfur dioxide gas is combined with water and
oxygen to make H₂SO₄.
• The company is expecting a shipment of 500
kg of SO₂ and wants to know how much
H₂SO₄ can be made from this shipment.
21
What are the steps to determine the
amount of sulfuric acid?
1.
2.
3.
22
Determining sales value
• They can then calculate how much money
they will make from the sale of their sulfuric
acid.
• Water and oxygen are readily available and
cheap, so the company uses an EXCESS of
these substances, which means they use more
than enough to react with the SO₂.
23
Sales
• If you can sell sulfuric acid for $10 per kg, how
much will be the total sales ($)?
24