Doc Brown's Chemistry - Chemical Calculations

On-line Quantitative Chemistry Calculations

Online practice exam chemistry CALCULATIONS and solved problems for KS4 Science GCSE/IGCSE CHEMISTRY and basic starter chemical calculations for A level AS/A2/IB courses * EMAIL query?comment or request for type of GCSE calculation?

The % by mass composition of a compound in terms of its constituent elements is calculated as follows:

• Example 4.1: Calculate the % of copper in copper sulphate, CuSO₄
  • Relative atomic masses: Cu = 64, S = 32 and O = 16
  • relative formula mass = 64 + 32 + 4x16 = 160
  • only one copper atom of relative atomic mass 64
  • % Cu = 64 x 100 / 160 = 40% copper by mass in the compound
• Example 4.2: Calculate the % of oxygen in aluminium sulphate, Al₂(SO₄)₃
  • Relative atomic masses: Al = 27, S = 32 and O = 16
  • relative formula mass = 2x27 + 3x(32 + 4x16) = 342
  • there are 4 x 3 = 12 oxygen atoms, each of relative atomic mass 16, giving a total mass of oxygen in the formula of 12 x 16 = 192
  • % O = 192 x 100 / 342 = 56.1% oxygen by mass in aluminium sulphate
• Example 4.3: Calculate the % of water in hydrated magnesium sulphate MgSO₄.7H₂O
  • Relative atomic masses: Mg = 24, S = 32, O = 16 and H = 1
  • relative formula mass = 24 + 32 + (4 x 16) + [7 x (1 + 1 + 16)] = 246
  • 7 x 18 = 126 is the mass of  water
  • so % water = 126 x 100 / 246 = 51.2 %
  • Note: The determination and calculation of the formula of a hydrated salt like MgSO₄.7H₂O is covered in Calculations section 14.4.
• Example 4.4: