Empirical and molecular formulae – IGCSE Chemistry exam questions with mark scheme

1Grade 6–74 marks

A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass. Its relative formula mass is 56.
[$A_r$: H = 1, C = 12]

(a) Calculate the empirical formula of the hydrocarbon. [3]

(b) Deduce its molecular formula. [1]

(a) C: 85.7 ÷ 12 (= 7.14) and H: 14.3 ÷ 1 (= 14.3)(1)
(a) divide by smallest: 1 : 2 (allow 1 : 2.00)(1)
(a) CH₂(1)
(b) C₄H₈ (56 ÷ 14 = 4; allow ECF from (a))(1)
Answer: (a) CH₂ (b) C₄H₈
Examiner tip: Divide by the Ar first, THEN by the smallest — dividing percentages by each other directly is the classic error.

2Grade 8–94 marks

Hydrated magnesium sulfate has the formula MgSO₄·xH₂O.

A student heats 4.92 g of the hydrated salt until all the water of crystallisation has been removed. The mass of anhydrous magnesium sulfate left is 2.40 g.
[$M_r$: MgSO₄ = 120, H₂O = 18]

Calculate the value of x. Show your working. [4]

mass of water = 4.92 − 2.40 = 2.52 (g)(1)
moles of MgSO₄ = 2.40 ÷ 120 = 0.0200 (mol)(1)
moles of H₂O = 2.52 ÷ 18 = 0.140 (mol)(1)
x = 0.140 ÷ 0.0200 = 7(1)
Answer: x = 7
Examiner tip: Use the mass of WATER lost (4.92 − 2.40), not the mass of the hydrated salt, when finding moles of H₂O.

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