Types of formula for organic molecules
3 marks · NormalA hydrocarbon has an empirical formula of CH2 and a molar mass of 56 g/mol.
Calculate its molecular formula. Show your working.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
C4H8
Common mistakes: Dividing by the wrong relative mass; Multiplying by wrong factor
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Finds relative mass of empirical formula CH2 (C = 12, H2 = 2, so 14).
M2 · 1 mark · Divides 56 by 14 to get 4, i.e. factor n = 4.
A1 · 1 mark · States molecular formula: C4H8. (follow-through)
Worked solution
Relative mass (Mr) of CH2 is 12 + 2×1 = 14.
Number of empirical units in molecule = 56 / 14 = 4.
Molecular formula = (CH2)₄ = C4H8.