Formulae and equations — return questions key

1. FR1 · Return

Model answer. (a) CO₂, NH₃ and CaCO₃. (b) SO₃. The molecular formula gives the number and type of different atoms in one molecule: one sulfur atom and three oxygen atoms. (c) 6, 12 and 6 all divide by 6, giving CH₂O. (d) Mg²⁺ carries +2, so it needs two NO₃⁻; the nitrate group stays in a bracket: Mg(NO₃)₂.

A full answer includes

• (a) CO₂, NH₃, CaCO₃.
• (b) SO₃: the number and type of atoms in one molecule, 1 sulfur and 3 oxygen.
• (c) CH₂O.
• (d) Mg(NO₃)₂.

Also accept

• (a) CO₂, NH₃, CaCO₃. (b) SO₃; atoms in one molecule. (c) CH₂O. (d) Mg(NO₃)₂.

Watch for

• “Ammonia is NH₄.” — NH₄⁺ is the ammonium ion; ammonia is NH₃.
• “The empirical formula is always the molecular formula halved.” — Divide by the largest number that goes into every subscript; it may be 2, 3, 6 or none at all.
• “One positive ion always joins one negative ion.” — Take just enough ions for the charges to add up to zero: Mg²⁺ needs two Cl⁻.

2. FR2 · Return

Model answer. Oxygen: both 2 and 3 go into 6, so 3O₂ and 2Al₂O₃. That makes 2 × 2 = 4 aluminium atoms on the right, so 4Al: 4Al + 3O₂ → 2Al₂O₃.

Lowest whole-number coefficients: 4Al(s) + 3O₂(g) → 2Al₂O₃(s)

A full answer includes

• 4Al + 3O₂ → 2Al₂O₃.

Also accept

• 4, 3, 2.

Watch for

• “Change a small number in a formula to make the atoms balance.” — Changing a subscript makes a different substance. Balance with numbers in front of the formulae.
• “Any numbers that balance are fine.” — Use the smallest whole numbers: if every number can be halved, halve them.

3. FR3 · Return

Model answer. (a) The sulfate ions stay dissolved on both sides, so they are spectators: Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s). The charge is +2 on each side. (b) Oxygen comes as O₂, so two CaO are made, which need two calcium atoms. Calcium is a solid metal, oxygen a gas and calcium oxide a solid: 2Ca(s) + O₂(g) → 2CaO(s).

A full answer includes

• (a) Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s).
• (b) 2Ca(s) + O₂(g) → 2CaO(s).

Also accept

• (a) Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s). (b) 2Ca(s) + O₂(g) → 2CaO(s).

Watch for

• “Keep every ion in the ionic equation.” — Ions that are the same on both sides are spectator ions; leave them out.
• “Oxygen, hydrogen, nitrogen and chlorine are written as single atoms, O, H, N and Cl.” — These elements exist as molecules of two atoms, so they are written O₂, H₂, N₂ and Cl₂.