MCQ practice · SSC Chemistry Board questions · Chapter 6: Concept of Mole and Chemical Counting
If the ratio of H, C and O atoms is 1 : 1 : 2 and the molecular mass is 90, which is the compound's molecular formula?
Think it through first, then pick an option.
- a HCO₂
- b CH₂O
- c H₂C₂O₄
- d C₆H₁₂O₆
Answer and explanation
Correct answer: c H₂C₂O₄
Empirical formula HCO₂ = 1 + 12 + 32 = 45; n = 90/45 = 2; molecular formula H₂C₂O₄.
Why the other options are wrong
- a HCO₂: HCO₂ is the empirical formula (mass 45); for 90, n = 2, so H₂C₂O₄.
- b CH₂O: CH₂O has H : C : O = 2 : 1 : 1, not the given 1 : 1 : 2.
- d C₆H₁₂O₆: C₆H₁₂O₆ matches neither the ratio nor the mass (180); the answer is H₂C₂O₄.
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