ZIMSEC O Level · Chemistry · Form 3, Form 4
Formulae and equations
Use valencies or ion charges: Ca²⁺ and Cl⁻ give CaCl₂; Al³⁺ and O²⁻ give Al₂O₃.
Balance equations by changing the numbers in front of formulae, never the formulae: 2Mg + O₂ → 2MgO. State symbols: (s), (l), (g), (aq).
The mole
- 1 mole contains 6 × 10²³ particles (the Avogadro constant).
- Moles = mass ÷ molar mass (n = m ÷ M).
- One mole of any gas occupies 24 dm³ at room temperature and pressure.
- Concentration (mol/dm³) = moles ÷ volume in dm³.
Worked example: How many moles are in 10 g of calcium carbonate, CaCO₃? (Ca = 40, C = 12, O = 16)
M = 40 + 12 + 3 × 16 = 100 g/mol. n = 10 ÷ 100 = 0.1 mol
Worked example: What mass of CO₂ forms when 0.1 mol CaCO₃ decomposes? CaCO₃ → CaO + CO₂
1 : 1 ratio, so 0.1 mol CO₂. Mass = 0.1 × 44 = 4.4 g
Percentage composition and empirical formula
% of element = (Ar × number of atoms ÷ Mr) × 100. For an empirical formula, divide each mass by Ar, then divide by the smallest answer.
Check yourself
- Balance: Al + O₂ → Al₂O₃.
- Find the volume of 0.5 mol of oxygen at r.t.p.
- 4 g of NaOH is dissolved to make 500 cm³ of solution. Find the concentration. (Na = 23, O = 16, H = 1)
Show answers
- 4Al + 3O₂ → 2Al₂O₃
- 12 dm³
- 4 ÷ 40 = 0.1 mol; 0.1 ÷ 0.5 = 0.2 mol/dm³
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