Question: A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass. Its relative molecular mass is 42. Find its molecular formula. (Ar: C = 12, H = 1)Step 1: find the empirical formula. Divide each percentage by the relative atomic mass.C: 85.7 ÷ 12 = 7.14 H: 14.3 ÷ 1 = 14.3 Divide both by the smallest value (7.14):C: 1 H: 14.3 ÷ 7.14 = 2.00 The empirical formula is CH2.Step 2: find the mass of one empirical formula unit.CH2 = 12 + (2 × 1) = 14 Step 3: find how many empirical units are in one molecule.n = Mr ÷ empirical formula mass = 42 ÷ 14 = 3 Step 4: multiply every number in the empirical formula by n.(CH2)3 = C3H6 Answer: the molecular formula is C3H6.Check: Mr of C3H6 = (3 × 12) + (6 × 1) = 42, which matches the value given.The value of n must always come out as a whole number. If it does not, recheck the empirical formula. The Mr is usually given in the question or found from the molecular ion peak in a mass spectrum.
Example Question - 2
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Specification: Edexcel Chemistry (8CH0), AS-level, Topic 5: Formulae, Equations and Amounts of Substance. Topic: Amount of Substance and Calculations.
Key terms in this lesson
- empirical formula
- The simplest whole number ratio of atoms of each element in a compound.
- molecular formula
- The actual number of atoms of each element in one molecule of a compound.
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All 39 lessons in Amount of Substance and Calculations · All Edexcel AS-level Chemistry topics