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:

1. calculate the formula or molecular mass of the compound
2. calculate the mass of the element in the compound, taking into account the number of atoms of the element in the compound formula
3. calculate (b) as a percentage of (a)

• 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: