The method and the results are on the balancing equations lab. Do the analysis yourself first, then check the answers below.
Quick answer: A formula shows the ratio of atoms or ions in a substance, and for ionic compounds the charges must cancel. These answers go with the Chemistry 9-10 lab on balancing equations: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
The answers use the sample observations. Whatever you observe, work out the precipitate by swapping partners between the ions, then check the formula against the ion charges before you balance. Include state symbols in every equation and check that charge balances in ionic equations.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS1-7
Question 1 Formulae of the precipitates
Copper(II) hydroxide: Cu²⁺ and OH⁻ give Cu(OH)₂. Iron(III) hydroxide: Fe³⁺ and OH⁻ give Fe(OH)₃. Magnesium hydroxide: Mg(OH)₂. Calcium carbonate: Ca²⁺ and CO₃²⁻ give CaCO₃.Cu(OH)₂, Fe(OH)₃, Mg(OH)₂, CaCO₃.
Question 2 Copper equation
Two OH⁻ are needed for each Cu²⁺, so 2NaOH is used and sodium sulfate forms.CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)
Question 3 Iron ionic equation
FeCl₃(aq) + 3NaOH(aq) → Fe(OH)₃(s) + 3NaCl(aq). Na⁺ and Cl⁻ are spectators. Charge: 3+ + 3(1−) = 0 on the right-hand side as well.Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)
Question 4 Spectator ions
The full equation is MgSO₄(aq) + 2NaOH(aq) → Mg(OH)₂(s) + Na₂SO₄(aq). Na⁺ and SO₄²⁻ are unchanged, so they are the spectators.Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s)
Question 5 No reaction
Swapping partners gives copper(II) chloride and iron(III) sulfate, and both are soluble, so all the ions stay in solution and nothing new forms.All possible products are soluble.
Question 6 Identify an unknown
A red-brown precipitate with hydroxide ions shows iron(III) ions, which form iron(III) hydroxide.Fe³⁺ and Fe(OH)₃.
Question 7 Improving reliability
Use equal volumes and a white background for all tubes, and clean the dropper between solutions so no contamination causes a false precipitate. Repeating each pair also checks consistency.Equal volumes, clean droppers and a white background.
