9701 · 2.3

Formulas — practice questions

Practice and worked examples for 9701 Formulas. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

A sample of a hydrocarbon was found to contain 85.7% carbon and 14.3% hydrogen by mass. Determine its empirical formula. ( $A_r$ : C = 12.0, H = 1.0)

Show solution outline

Assume 100g sample & find moles:
- Mass of C = 85.7 g; Mass of H = 14.3 g
- Moles of C = Mass / $A_r$ = 85.7 g / 12.0 g mol⁻¹ = 7.142 mol
- Moles of H = Mass / $A_r$ = 14.3 g / 1.0 g mol⁻¹ = 14.3 mol
Find the simplest ratio:
- Divide by the smallest number of moles (7.142):
- C: 7.142 / 7.142 = 1
- H: 14.3 / 7.142 = 2.002 ≈ 2
Write the empirical formula:
- The simplest whole-number ratio of C:H is 1:2.
- The empirical formula is CH₂.

The compound from the previous example (empirical formula CH₂) has a relative molecular mass ( $M_r$ ) of 56.0. Determine its molecular formula.

Show solution outline

Calculate the empirical formula mass:
- Mass of CH₂ = 12.0 + (2 × 1.0) = 14.0
Find the integer multiple 'n':
- n = (Relative Molecular Mass) / (Empirical Formula Mass)
- n = 56.0 / 14.0 = 4
Determine the molecular formula:
- Molecular Formula = (Empirical Formula) $_n$
- Molecular Formula = (CH₂)₄ = C₄H₈