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?

When elements and compounds react to form new products, mass cannot be lost or gained - "The Law of Conservation of Mass" - states that mass cannot be created or destroyed, but changed into different forms, so in a chemical change the total mass of reactants must equal the total mass of products. By using this law, together with atomic and formula masses, you can calculate the quantities of reactants and products involved in a reaction and the simplest formula of a compound (see also Ex 5. shows how to get to a compound's formula).

NOTE: (1) the symbol equation must be correctly balanced to get the right answer! (2) There are good reasons why, when doing a real chemical preparation-reaction to make a substance you will not get 100% of what you theoretically calculate. See discussion in section 14.2

• Example 3.1: Magnesium + Oxygen ==> Magnesium oxide
  • 2Mg + O₂ ==> 2MgO (atomic masses required: Mg=24 and O=16)
  • think of the ==> as an = sign, so the mass changes in the reaction are:
  • (2 x 24) + (2 x 16) = 2 x (24 + 16)
  • 48 + 32 = 2 x 40 and so 80 mass units of reactants = or produces 80 mass units of products (you can work with any mass units such as g, kg or tonne (1 tonne = 1000 kg)
• Example 3.2: iron + sulphur ==> iron sulphide
  • Fe + S ==> FeS (atomic masses: Fe = 56, S = 32)
  • If 59g of iron is heated with 32g of sulphur to form iron sulphide, how much iron is left unreacted? (assuming all the sulphur reacted)
  • From the atomic masses, 56g of Fe combines with 32g of S to give 88g FeS.
  • This means 59 - 56 = 3g Fe unreacted.
• Example 3.3: When limestone (calcium carbonate) is strongly heated, it undergoes thermal decomposition to form lime (calcium oxide) and carbon dioxide gas.
  • CaCO₃ ==> CaO + CO₂ (relative atomic masses: Ca = 40, C = 12 and O = 16)
  • Calculate the mass of calcium oxide and the mass of carbon dioxide formed by decomposing 50 tonnes of calcium carbonate.
  • (40 + 12 + 3x16) ==> (40 + 16) + (12 + 2x16)
  • 100 ==> 56 + 44, scaling down by a factor of two, 50 ==> 28 + 22
  • so decomposing 50 tonnes of limestone produces 28 tonnes of lime and 22 tonnes of carbon dioxide gas.
• Example 3.4: