The ratio of carbon atoms of hydrogen atoms in a hydrocarbon is 1:2. If its molecular mass is 56, what is its molecular formula?

Analyze the question: The ratio of C atoms to H atoms is 1:2, so the empirical formula is CH₂. The molecular mass is 56, and we need the molecular formula.

Concept: Molecular formula = (empirical formula)ₙ, where

n = molecular mass ÷ empirical formula mass

Solution:

1. Empirical formula = CH₂
2. Empirical formula mass = 12 + (2 × 1) = 14
3. n = 56 ÷ 14 = 4
4. Molecular formula = (CH₂)₄ = C₄H₈

Check: C₄H₈ has a mass of (4 × 12) + (8 × 1) = 48 + 8 = 56 ✓

Checking the other options:

• A. C₃H₆ = 42 ✗
• C. C₂H₄ = 28 ✗
• D. CH₂ = 14 ✗

Tip: For these questions, find the empirical formula mass first, then divide the molecular mass by it to get n. You can also test the options quickly by adding up their masses. Only the option that gives 56 is correct, and it must also keep the 1:2 ratio.