# Stoichiometry & the mole concept – O Level

## SKIM OVER

#### Mole

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

no of moles = no of particles ÷ 6×1023

#### 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)

#### 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%

#### 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

#### 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

#### 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 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

#### 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

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

The extent and efficiency of converting reactants to products.

% yield = expected yield ÷ theoretical yield × 100%

#### 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%