Doc Brown's Chemistry - GCSE/IGCSE/GCE (basic A level) O Level  Online 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" definition 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 Section 5. which shows how to use this law 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 (see the diagram at the top of the page!)
  • 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: