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InkFlaskCambridge IGCSE
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Answers · Formulae and equations

Formulae and equations — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. FK1 · Final check

Model answer. (a) CH4, NaOH and N2 (nitrogen is a molecule of two atoms). (b) The molecular formula of a compound is the number and type of different atoms in one molecule. (c) C3H6O. (d) 6 and 12 both divide by 6: CH2. (e) The smallest total both charges make is 6: two Fe3+ (+6) and three O2− (−6), Fe2O3.

A full answer includes

  • (a) CH4, NaOH, N2.
  • (b) The number and type of different atoms in one molecule.
  • (c) C3H6O.
  • (d) CH2.
  • (e) Fe2O3: 2 × (+3) = +6 and 3 × (−2) = −6.

Also accept

  • (a) CH4, NaOH, N2. (b) Atoms in one molecule. (c) C3H6O. (d) CH2. (e) Fe2O3.

Watch for

  • “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.
  • “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. FK2 · Final check

Model answer. (a) propane + oxygen → carbon dioxide + water. (b) Carbon: 3, so 3CO2. Hydrogen: 8, so 4H2O. Oxygen on the right: 3 × 2 + 4 × 1 = 10 atoms, which is 5O2. At room temperature carbon dioxide is still a gas, but water is a liquid: C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l).

A full answer includes

  • (a) propane + oxygen → carbon dioxide + water.
  • (b) C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l).

Also accept

  • (a) Names of all four substances with + and →. (b) C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l).

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.
  • “Some atoms are used up in the reaction, so the two sides needn't match.” — Atoms are not made or destroyed in a reaction, only rearranged, so each element has the same number of atoms on both sides.
  • “The water is (g) because the propane burned in a hot flame.” — The products were cooled to room temperature, so the water is a liquid, (l).

3. FK3 · Final check

Model answer. (a) MgCl2 has two chlorine atoms, so 2HCl; then hydrogen is 2 and 2. The ribbon is solid, the acid and the magnesium chloride solution are aqueous, and hydrogen is a gas: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g). (b) Split the dissolved compounds: Mg(s) + 2H+(aq) + 2Cl−(aq) → Mg2+(aq) + 2Cl−(aq) + H2(g). The chloride ions are spectators, so Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g); the charge is +2 on each side.

A full answer includes

  • (a) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g).
  • (b) Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g).

Also accept

  • (a) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g). (b) Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g).

Watch for

  • “Keep every ion in the ionic equation.” — Ions that are the same on both sides are spectator ions; leave them out.
  • “Only the atoms must balance in an ionic equation.” — The total charge must be the same on both sides too.