Mole & Equations

• You can use moles to calculate masses in reactions
• The big numbers in front of the chemical formulas of the reactants and products tell you how many moles of each substance takes part or is formed during the reaction e.g:

  Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)

  > In this reaction, 1 mole of magnesium and 2 moles of hydrochloric acid react together to form 1 mole of magnesium chloride and 1 mole of hydrogen gas
• You can balance equations using reacting masses
• If you know the masses of the reactants and products that took part in a reaction, you can work out the balanced symbol equation for the reaction:

  • Divide the mass of each substance by its relative formula mass to find the number of moles
  • Divide the number of moles of each substance by the smallest number of moles in the reaction
  • If any of the numbers aren't hole numbers, multiply all the numbers by the same amount so that they all become whole numbers
  • Write the balanced symbol equation for the reaction by putting these numbers in front of the chemical formulas
• Example

  8.1 g zinc oxide (ZnO) reacts completely with 0.60 g or carbon to form 2.2 g of carbon dioxide and 6.5 g of zinc. Write a balanced symbol equation for this reaction. Ar (C) = 12, Ar (O) = 16, Ar (Zn) = 65.

  1)Work out Mr for each of the substances in the reaction

  > ZnO: 65 + 16 = 81 C: 12 CO2: 12 + (2 x 16) = 44 Zn: 65

  2) Divide the mass of each substance by its Mr to calculate how many moles of each substance reacted or were produced

  > ZnO: 8.1/81 = 0.10 mol CO2: 2.2/44 = 0.050 mol

  > C: 0.60/12 = 0.050 mol Zn: 6.5/65 = 0.10 mol
• Example

  8.1 g zinc oxide (ZnO) reacts completely with 0.60 g or carbon to form 2.2 g of carbon dioxide and 6.5 g of zinc. Write a balanced symbol equation for this reaction. Ar (C) = 12, Ar (O) = 16, Ar (Zn) = 65.

  3) Divide by the smallest number of moles, which is 0.050

  > ZnO: 0.10/0.050 = 2.0 CO2: 0.050/0.050 = 1.0

  > C: 0.050/0.050 = 1.0 Zn: 0.10/0.050 = 2.0

  4) The numbers are all whole numbers, so you can write out the balanced symbol equation straight away

  > 2ZnO + C -> CO2 + 2Zn