Section 7.3 – Using Chemical Formulas
 They allow chemists
to calculate a number
of characteristic
values for a given
compound.
 Formula mass
 Molar mass
 Percentage
composition
Formula Mass
 The formula mass of any
molecule, formula unit,
or ion is the sum of the
average atomic masses
of all the atoms
represented in its
formula.
 Reported in amu (atomic
mass units).
Example – Water (H2O)
 mass H atom = 1.0079 amu
 mass O atom = 15.9994 amu
 Mass of 1 water molecule =
(2 x 1.0079) + (1 x 15.9994)
= 18.0152 amu/molecule
Your turn #1
 What is the formula mass
of sulfuric acid?
 H2SO4
 2xH + 1xS + 4xO =
2(1.00794) + 1(32.066) +
4(15.9994) = 98.079 amu
Your turn #2
 What is the formula mass
of calcium nitrate?
 Ca(NO3)2
 1xCa + 2xN + 6xO =
1(40.078) + 2(14.00674) +
6(15.9994) = 164.088 amu
Remember The Mole?
 The molar mass of
a substance is the
mass in grams of
one mole (i.e., the
mass in grams of
6.022 x 1023
particles).
Molar Mass
Calculated by summing the
masses of the elements
present in one mole of the
substance (6.022 x 1023
particles)
A mole of molecules,
formula units, or ions of a
compound.
Reported in grams.
How many moles of atoms are there?
How many moles of atoms
of each element are there
in aluminum sulfide?
Al2S3
One mole of aluminum
sulfide contains 2 moles of
Al, and 3 moles of S
How many moles of atoms are there?
How many moles of atoms
of each element are there
in barium hydroxide?
Ba(OH)2
1 mol Ba
2 mol O
2 mol H
Molar Mass – Water (H2O)
 1 mole H atoms = 1.0079
grams
 1 mole O atoms = 15.9994
grams
 1 mole water molecules =
(2 x 1.0079) + (1 x 15.9994)
= 18.0152 g/mole H2O
Molar Mass – Example
 What is the mass of one
mole of sodium nitrate?
 NaNO3
 1 mol Na(22.989768g/mol)
+ 1 mol N(14.00674g/mol)
+ 3mol O(15.9994g/mol) =
84.9947 g
 Molar mass is 84.9947
g/mol
Notice
 The molar mass and
the formula mass are
numerically equal;
 formula mass is
reported in amu
 Molar mass is
reported in
grams/mol
Molar Mass As A Conversion Factor
 See p. 224
 Can use molar mass
to convert
 grams  moles 
molecules
 Molecules  moles
 grams
Examples
 What is the mass in
grams of 3.04 mol
of ammonia vapor,
NH3?
 What is the mass of
0.257 mol of
calcium nitrate?
Examples
 How many moles of
compound are there
in 6.60g of
ammonium sulfate?
 How many
molecules are there
is 25.0g of sugar
(C12H22O11)?
Percentage Composition
 Often useful to
know the percentage
by mass of each
element in a
compound.
 There are two ways
to determine it:
Determine % composition by Mass
 Mass of element in a
sample divided by the
mass of the sample
multiplied by 100. The
percentage will be the
same regardless of the
sample size.
Percent composition by moles (Easier)
 Calculate how many
grams of the
element are present
in 1 mole of the
compound and
divide by molar
mass

𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 1 𝑚𝑜𝑙𝑒
𝑥100%
𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑
Example
 Find the percentage composition of copper (I)
sulfide
 Cu2S




63.55 𝑔 𝐶𝑢
2 mol Cu x
= 127.1 𝑔
1 𝑚𝑜𝑙 𝐶𝑢
32.07 𝑔 𝑆
1 mol S x
= 32.07 𝑔 𝑆
1 𝑚𝑜𝑙 𝑆
127.1 𝑔 𝐶𝑢
𝑥100 = 79.84% 𝐶𝑢
159.2 𝑔 𝐶𝑢𝑆2
32.07 𝑔 𝑆
𝑥100
159.2 𝑔 𝐶𝑢𝑆2
= 20.14% S
𝐶𝑢
Info. You Will Need For Later
 Hydrates are salts
that crystallize from
a water solution and
trap water (known
as water of
hydration) in the
crystal structure.
Na2CO3•10H2O
 This means that 10
moles of water are
trapped within each
mole of the sodium
carbonate salt.
Assignment –
Due Friday
 7.3 Worksheet
 Chapter 7 Element
Handbook Problems
 7.3 Textbook
Problems