Stoichiometry & the mole concept – O Level





1 mole is 6×1023 particles. Stoichiometry means the mole ratio of substances in a reaction.

no of moles = no of particles ÷ 6×1023

Comparing the mass of a molecule to 1/12th of a carbon-12 atom

Relative Molecular Mass

The mass of a molecule compared to 1/12th of a carbon-12 atom.

Mr of AnBm = (n × Ar of A) + (m × Ar of B)

Mass balance

Molar Mass

The mass of 1 mol of a substance, numerically equal to the Ar or Mr.

molar mass = mass in g ÷ no of moles


Percentage Mass

The percentage by mass of a compound that is made up of an element.

% mass of A in AnBm= nAr ÷ Mr × 100%

Empirically determining the ratio of atoms in a compound in the lab

Empirical Formula

The simplest ratio of atoms of each element in a compound.

1. Write the masses of each element
2. Divide by Ar
3. Divide by smallest number

Atoms in a molecule

Molecular Formula

Shows the number of atoms of each element in one simple molecule.

1. Find n = Mr ÷ mass of empirical formula
2. Multiply numbers in empirical formula by n

Balloon containing gas

Molar Volume

A mole of gas occupies 24 dm3 of volume at room temperature and pressure (rtp).

no of moles = gas volume in dm3 ÷ 24

Solution in a beaker

Solution Concentration

The number of moles or mass of solutes per dm3 of solvent.

concentration = no of moles ÷ volume in dm3
concentration = mass ÷ volume in dm3

Burette for titration


Carefully adding exact amount of one solution to completely react with another.

1. Find no of moles of titrant = concentration × volume
2. Use mole ratio to find no of moles of unknown
3. Find conc of unknown = no of mol ÷ volume

Limiting reactant limits the amount of product as the reaction stops when it is completely used up

Limiting Reactant

The reactant that is completely used up, limiting the amount of product.

1. Find the no of moles of reactants A and B provided
2. Find the no of moles of B needed if A were used up
3. A is limiting if there is less B needed than provided

Percentage yield is a gauge of the efficiency and extent of a reaction

Percentage Yield

The extent and efficiency of converting reactants to products.

% yield = expected yield ÷ theoretical yield × 100%

Percentage purity shows how pure something is

Percentage Purity

The percentage by mass of an impure mixture that comprises the pure substance.

% purity = mass of pure substance ÷ mass of impure mixture × 100%