Molecular Formula from Percentage Composition – JEE Main PYQ

JEE Main Previous Year Question

An organic compound has 42.1% carbon, 6.4% hydrogen and remainder is oxygen. If its molecular weight is 342, then its molecular formula is :

C₁₄H₂₀O₁₀

C₁₂H₂₂O₁₁

C₁₂H₂₀O₁₂

C₁₁H₁₈O₁₂

Step 1: Assume 100 g of compound

Carbon = 42.1 g
Hydrogen = 6.4 g
Oxygen = 100 − (42.1 + 6.4) = 51.5 g

Step 2: Convert mass into moles

Moles of C = 42.1 / 12 ≈ 3.51
Moles of H = 6.4 / 1 = 6.4
Moles of O = 51.5 / 16 ≈ 3.22

Step 3: Find simplest whole number ratio

Divide by smallest value (3.22):
C : 1
H : 2
O : 1

Empirical formula = CH₂O

Step 4: Find molecular formula

Empirical formula mass = 30
Molecular mass = 342
Multiplier = 342 / 30 = 11

Molecular formula = C₁₂H₂₂O₁₁

Correct Answer: C₁₂H₂₂O₁₁