3.2 Use of Amount of Substance in Relation to Masses of Pure Substance

Moles

Moles = Mass (g) / Mr

The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant

Avogadro’s constant: 6.02 x 10^23

Equations

Use moles to balance equations

Calculate masses of reactants and products from balanced equations

E.g. 2Li + F2 → 2LiF
2 moles of lithium reacts with one mole of fluorine to produce 2 moles of lithium fluoride

Method

  1. Calculate moles of known reactant/product
  1. Determine ratio of known reactant/product to unknown reactant/product
  1. Find mass of unknown reactant/product

Equal number of atoms of each element on each side of the equation

Concentration

Describes the mass of a substance in a certain volume of solution

Can be measured in mass per unit volume

Concentration (g/dm3) = Mass (g) ÷ volume (dm3)

Limiting reactants

Reactant which gets used up, causing the reaction to stop

Any other reactants are in excess

Determines the amount of product

Calculate the mass of product formed using the mass of the limiting reactant and the balanced equation

Amount of product formed is directly proportional to amount of limiting reactant

Method

  1. Calculate moles of limiting reactant
  1. Determine ratio of limiting reactant to product
  1. Find mass of product