Chapter 12 Chemical Quantities
12.2 Using Moles
Warm-up: Making a Sandwich
How Many Sandwiches?
What is Stoichiometry?
Stoichiometry is the study of quantitative relationships
between amounts of reactants used and products formed
by a chemical reaction.
A balanced chemical equation is needed to solve any stoichiometry problem
Practical Applications of Stoichiometry
In a spacecraft, the carbon dioxide exhaled by
astronauts can be removed by its reaction with lithium
hydroxide, LiOH, according to the following chemical
equation.
CO2(g) + 2LiOH(s)
Li2CO3(s) + H2O(l)
How many moles of lithium hydroxide are required to
react with 20 mol of CO2, the average amount exhaled
by a person each day?
Strategy – Relate everything to the mole
The mole is the common thread
that links reactants to products.
1CO2(g) + 2LiOH(s)
1Li2CO3(s) + 1H2O(l)
Mole Ratios
1molCO2
1molCO2
2molLiOH 1molLi2CO3
1molCO2 2molLiOH
1molH 2O 1molLi2CO3
2molLiOH
1molH 2O
The Mole Super Highway
Practice Problems – Mole to Mole
Balanced
Chemical Equation
How many moles of water are formed when 3.8 moles of Hydrogen react with an
excess of Oxygen?
2𝐻2 + 𝑂2 → 2𝐻2 O
How many moles of oxygen are needed to fully combust 1.6 moles of ethane(C2H6)?
__ C2 H 6  __ O2  __ CO2  __ H 2O
Warm-up
Felix has six of the same Charazard cards. For every 3
Charazard cards that he trades in, he can get one Arcanine
cards. Each Arcanine card is valued at $2.50. How much
money can Felix make from a trade if he trades in all of his
Charazard cards?
3
1
Practice Problems – Mole to Mass
How many grams of water are formed when 4.5 moles of oxygen react with an
excess of hydrogen?
2𝐻2 + 𝑂2 → 2𝐻2 O
How many moles of oxygen are needed to produce 15.6g of carbon dioxide?
2C2 H 6  7O2  4CO2  6 H 2O
Practice Problems – Mass to Mass
How many grams of water are formed when 18.6 grams of oxygen react with an
excess of hydrogen?
2𝐻2 + 𝑂2 → 2𝐻2 O
How many grams of ethane are needed to produce 23.4g of water?
2C2 H 6  7O2  4CO2  6 H 2O
Warm-up
At STP, which of the following would have the greatest
volume?
1mole = He
1mole = NO
1mole = CH4
Practice Problems – Volume at STP
How many liters of oxygen are required to react with 5.6 liters of hydrogen to
produce water at STP?
2𝐻2 + 𝑂2 → 2𝐻2 O
Solving for Volume without STP Conditions

When a chemical reaction is carried out under conditions
other than STP, the ideal gas law equation must be used.
PV=nRT is the Ideal Gas Law Equation where….



P = pressure
V = volume
n = moles
8.31 𝐿.𝑘𝑃𝑎
𝐾.𝑚𝑜𝑙

R = gas constant =

T = temperature in Kelvin
=
0.082 𝐿.𝑎𝑡𝑚
𝐾.𝑚𝑜𝑙
Practice Problems – Ideal Gas Law
PV=nRT
R = gas constant =
8.31 𝐿.𝑘𝑃𝑎
𝐾.𝑚𝑜𝑙
=
0.082 𝐿.𝑎𝑡𝑚
𝐾.𝑚𝑜𝑙
2C2 H6  7O2  4CO2  6H 2O
What volume of carbon dioxide can be produced from 1.2 moles
of ethane gas at 200K and 0.65 atm?
Warm-up

You mix together the ingredients to make a batch of
chocolate chip cookies. The recipe says that the yield
should be 3 dozen cookies. You only get 30 cookies.
What percentage of the expected yield did you get?
Percent Yield
Stoichiometric calculations provide a theoretical yield or
maximum amount of product that can be produced
from a given amount of reactants.
The actual yield is the amount that is actually produced
when a reaction is carried out.
The percent yield reflects the ratio of the theoretical
and actual yields
ActualYiel d ( Experiment)
PercentYield 
x100
TheoreticalYield ( Stoichiometry)
Practice Problem - Percent Yield
When 4.3 grams of sodium are combined with aluminum
nitrate in the following equation, 1.52g of aluminum are
produced. What is the percent yield of aluminum?
3𝑁𝑎 + 𝐴𝑙(𝑁𝑂3 )3 → 𝐴𝑙 + 3𝑁𝑎𝑁𝑂3
Warm-up

What percentage of the students in this classroom are girls?
Percent Composition by Mass
The total mass of each
individual element in a
compound divided by the
total mass of the compound
What is the percent composition of Mg(BrO4)2?
Remember how to calculate molar mass…..
1(mass Mg) + 2(mass of Br) + 8(mass of oxygen) = Mass of Mg(BrO4)2
Warm-up

Simplify the following fractions.
30
45
4
28
36
12
Empirical Formula
Chemical formula which contains the lowest whole
number ratio of atoms
Ionic formulas always contain the lowest whole number ratios
Covalent compounds do not always contain the lowest whole number ratios
What is the Empirical Formula of the following compounds?
C6H12O6
H2O2
C3H9O6
Determining the Empirical Formula of a
Compound
What is the empirical formula for a
compound which contains 25.9%
nitrogen and 74.1% oxygen by mass?
NxOy
1. Covert % to mass (if needed)
Assume a 100g sample. In that sample 25.9 g is nitrogen and 74.1g is oxygen
2. Convert mass to moles
 1mole 
 1mole 

  4.63molO
74
.
1
gO


25.9 gN 
 1.85molN

 16 g 
 14 g 
3. Divide each molar quantity by the
lowest number of moles
N 1.85 O4.63
1.85
4. Multiply by the smallest number
needed to create a whole number
ratio (if needed)
N1.85O4.63
N1O2.5
1.85
N1x2O2.5x2
N2O5
Warm-up
Which of the following are empirical
formulas?
C2H8N2
H3PO4
C2H8
CH4
C8H12O4
Li2SO4
Molecular Formulas
Many chemicals can have the same empirical formula because atoms can share
electrons in many ways.
𝐶6 𝐻12 𝑂6
Glucose
𝐶2 𝐻4 𝑂2
Acetic Acid
Empirical Formula = 𝐶𝐻2 O
Simple Molecular Problems
What is the molecular formula of a compound
with an empirical formula of CH2O and a
molecular mass of 90g/mol?
1. Determine the empirical mass
CH2O =1(12g)+2(1g)+1(16g) = 30g
2. Divide the molecular mass by the
empirical mass to determine how
much larger the mass of the
molecule is compared to the
empirical formula
3. Multiple the subscripts of each
element in the empirical formula by
the factor determined in #2
C1x3H2x3O1x3
C3H6O3
Longer Molecular Problems
1. Determine the empirical formula
2. Given the molecular mass, determine the
molecular formula
Determine the molecular formula of a compound
that has a mass of 34g and contains 94.1%
oxygen and 5.9% hydrogen by mass.