Moles Revision Notes
• The term mole is used as a unit for the amount of substance. One mole of a substance
is it’s relative atomic mass, or relative formula mass, in grams.
Calculating moles from mass (using Mr)
moles
Mass
mamaismass
m
grams Mass moles x molar mass
Molar mass
Moles
Mr
n
g/mol or
mol
g.mol
Calculating empirical and molecular formulae
The empirical formula of a compound shows the ratio of each element present, using the
smallest whole number of atoms.
The molecular formula of a compound gives the actual numbers of atoms of each element
in a molecule.
How to calculate empirical formula:
1. Find masses or percentages in the info
2. Look up Mr values
3. Divide the mass by the Mr to get the mole ratio
4. If needed, divide by the smallest to get a whole number ratio
How to calculate molecular formula:
The molecular formula can be found by dividing the Mr of the compound by the relative
mass of the empirical formula to get the scaling up factor.
E.g. What is the molecular formula of hydrogen peroxide given that it’s empirical formula is
HO and the Mr is 34?
1. Relative mass of empirical formula = 17
2. Divide Mr by mass of empirical formula to get multiple - 34/17 = 2
• The term mole is used as a unit for the amount of substance. One mole of a substance
is it’s relative atomic mass, or relative formula mass, in grams.
Calculating moles from mass (using Mr)
moles
Mass
mamaismass
m
grams Mass moles x molar mass
Molar mass
Moles
Mr
n
g/mol or
mol
g.mol
Calculating empirical and molecular formulae
The empirical formula of a compound shows the ratio of each element present, using the
smallest whole number of atoms.
The molecular formula of a compound gives the actual numbers of atoms of each element
in a molecule.
How to calculate empirical formula:
1. Find masses or percentages in the info
2. Look up Mr values
3. Divide the mass by the Mr to get the mole ratio
4. If needed, divide by the smallest to get a whole number ratio
How to calculate molecular formula:
The molecular formula can be found by dividing the Mr of the compound by the relative
mass of the empirical formula to get the scaling up factor.
E.g. What is the molecular formula of hydrogen peroxide given that it’s empirical formula is
HO and the Mr is 34?
1. Relative mass of empirical formula = 17
2. Divide Mr by mass of empirical formula to get multiple - 34/17 = 2
