This set has ten original questions, ordered from easier to harder, on the skills in the formulas and equations module. All questions and scenarios are invented teaching examples written for practice, not past-paper items. The reactions are at concept level only, for working on paper.
Write each answer on paper first, then open the worked answer. Read the reasoning even when you were right.
Questions 1 to 3: formulas from ion charges
1. Write the formula for sodium sulfide, given Na⁺ and S²⁻.
Show answer
Two Na⁺ (2+) balance one S²⁻ (2−). Na₂S. Check: 2 × 1+ = 2+ and 2−.
2. Write the formula for calcium nitrate, given Ca²⁺ and NO₃⁻.
Show answer
One Ca²⁺ (2+) needs two NO₃⁻ (2 × 1− = 2−). The nitrate group repeats, so it needs brackets: Ca(NO₃)₂.
3. Write the formula for aluminium hydroxide, given Al³⁺ and OH⁻.
Show answer
One Al³⁺ (3+) needs three OH⁻ (3−). Brackets are needed around OH: Al(OH)₃. Check: 1 Al, 3 O, 3 H.
Questions 4 to 7: balancing
4. Balance: Na + Cl₂ → NaCl
Show answer
Cl₂ gives 2 Cl, so put 2 before NaCl. That gives 2 Na, so put 2 before Na.
2Na + Cl₂ → 2NaCl. Check: Na 2 = 2, Cl 2 = 2.
5. Balance: Fe + Cl₂ → FeCl₃
Show answer
Cl: 2 on the left and 3 on the right. The lowest common multiple is 6, so use 3Cl₂ and 2FeCl₃. Then Fe: 2 on the right, so 2Fe.
2Fe + 3Cl₂ → 2FeCl₃. Check: Fe 2 = 2, Cl 6 = 6.
6. Balance: C₂H₆ + O₂ → CO₂ + H₂O
Show answer
Carbon: 2 C on the left, so 2CO₂. Hydrogen: 6 H, so 3H₂O. Oxygen on the right: 4 + 3 = 7, which is odd. O₂ gives even numbers, so double everything: 2C₂H₆ gives 4CO₂ and 6H₂O. Oxygen now: 8 + 6 = 14, so 7O₂.
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. Check: C 4 = 4, H 12 = 12, O 14 = 14.
7. Balance: Al(OH)₃ + H₂SO₄ → Al₂(SO₄)₃ + H₂O
Show answer
Al₂(SO₄)₃ has 2 Al, so 2Al(OH)₃. It has 3 SO₄, so 3H₂SO₄. Hydrogen on the left: 2 × 3 + 3 × 2 = 12, so 6H₂O on the right (12 H).
2Al(OH)₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 6H₂O. Check: Al 2, S 3, H 12, O 6 + 12 = 18 on the left and 12 + 6 = 18 on the right.
Questions 8 to 9: state symbols and word equations
8. Magnesium ribbon is added to dilute sulfuric acid. Bubbles of gas form and a solution of magnesium sulfate remains. Write the balanced equation with state symbols.
Show answer
Balanced: Mg + H₂SO₄ → MgSO₄ + H₂ (1 : 1 : 1 : 1). Evidence: “ribbon” is (s), “dilute sulfuric acid” is (aq), “solution of magnesium sulfate” is (aq), “bubbles of gas” is (g).
Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)
9. Sodium hydroxide solution is mixed with dilute nitric acid, forming a solution of sodium nitrate and water. Write the word equation.
Show answer
An acid with an alkali forms a salt and water.
sodium hydroxide + nitric acid → sodium nitrate + water
Question 10: ionic equation
10. Sodium carbonate solution is mixed with calcium chloride solution. A white precipitate of calcium carbonate forms. Write the full equation with state symbols, the ionic equation and name the spectator ions.
Show answer
Full: Na₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + 2NaCl(aq). Check: Na 2, C 1, O 3, Ca 1, Cl 2 on both sides.
Split the (aq) substances: 2Na⁺ + CO₃²⁻ + Ca²⁺ + 2Cl⁻ → CaCO₃(s) + 2Na⁺ + 2Cl⁻. Cancel 2Na⁺ and 2Cl⁻.
Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s). Spectator ions: Na⁺ and Cl⁻. Charge: 2+ and 2− gives 0 on the left, 0 on the right.
If you got these wrong
| What went wrong | Go back to |
|---|---|
| Wrong number of ions, or a missing bracket (Q1 to 3) | Construct a formula from ion charges |
| Atoms do not match, or a subscript was changed (Q4 to 7) | Balance atoms without changing subscripts |
| A state symbol missing or unjustified (Q8, Q10) | Include state symbols from supplied evidence |
| Reactants and products in the wrong place (Q9) | Write a word equation from a described change |
| Ions not cancelled, or a solid split up (Q10) | Identify a spectator species in an ionic equation |
How should I check my working?
Use the equation balance reasoning trainer to recount atoms, and the mole and equation-ratio tutor to see what the coefficients mean as ratios. Then record each error type in the mistake log and retest queue, so the same slip gets a fresh question later.
When you repeat the same mistake after reviewing the lesson, the cause is usually one small habit. Finding it is easier with a teacher watching your written working, as in our online one-to-one Chemistry tuition. Move on afterwards to relative masses and amounts.