Types of formula for organic molecules
2 marks · EasyA compound has the empirical formula CH2. Its relative molecular mass is 56. Calculate its molecular formula. Show your working.
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C4H8
Common mistakes: C2H4; C3H6
Show mark scheme and worked solution
Marking points
M1 · 1 mark · 1 mark for calculating the molar mass of empirical unit: (C + 2H) = 12 + (2 × 1) = 14
A1 · 1 mark · 1 mark for correct molecular formula: (56/14 = 4, so empirical unit × 4 = C4H8) (follow-through)
Worked solution
Empirical unit: C = 12, H = 1, so CH2 = 12 + 2 = 14.
Number of units in molecular formula = 56 / 14 = 4.
Molecular formula = (CH2) × 4 = C4H8.