The questions are on the balancing equations test. Take it under real conditions before reading the mark scheme.
Quick answer: A formula shows the ratio of atoms or ions in a substance, and for ionic compounds the charges must cancel. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 9-10 test on balancing equations, from write formulae to charge on zinc, with the equivalent answers to accept.
Use the model answers and marking points to score your paper out of 20. For balancing marks, an equation counts only if every coefficient is correct. Note any wrong formulae first, because a wrong formula makes an equation impossible to balance.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS1-7
Question 2 Balance the equations (3 marks)
- N₂ + 3H₂ → 2NH₃
- 2Al + 3Cl₂ → 2AlCl₃
- 2KClO₃ → 2KCl + 3O₂
Marking: 1 mark for each fully correct equation.
Question 3 State symbols (2 marks)
(aq) means dissolved in water. The symbol for a gas is (g).Marking: 1 mark each.
Question 4 Precipitate of silver bromide (3 marks)
- AgNO₃(aq) + NaBr(aq) → AgBr(s) + NaNO₃(aq)
- Na⁺ and NO₃⁻
- Ag⁺(aq) + Br⁻(aq) → AgBr(s)
Marking: 1 mark each part.
Question 5 Changing a subscript (2 marks)
MgO₂ is a different substance from magnesium oxide, so a subscript must not be changed. The correct equation is 2Mg + O₂ → 2MgO.Marking: 1 mark for the explanation, 1 mark for the correct equation.
Question 6 Burning aluminium (3 marks)
- 10.2 − 5.4 = 4.8 g.
- The law of conservation of mass.
Marking: 1 mark for the method, 1 mark for 4.8 g, 1 mark for the law.
Question 7 Formula from ion cards (2 marks)
2 Fe³⁺ give 6+ and 3 SO₄²⁻ give 6−, so the formula is Fe₂(SO₄)₃.Marking: 1 mark for the formula, 1 mark for the charge calculation.
Question 8 Charge on zinc (2 marks)
Two nitrate ions carry 2 × 1− = 2−, so the zinc ion must carry 2+. The ion is Zn²⁺.Marking: 1 mark for the nitrate charge total, 1 mark for 2+.
