A compound has an empirical formular CH2O and molecular mass of 90. [H = 1.00, C = 12.0, O = 16.0]

Step 1: Analyze the question We are given the empirical formula (CH₂O) and the molecular mass (90). We need to find the molecular formula.

Step 2: Concept Molecular formula = (empirical formula)ₙ, where

n = molecular mass ÷ empirical formula mass

Step 3: Solution

Empirical formula mass of CH₂O: = 12.0 + (2 × 1.00) + 16.0 = 30.0

n = 90 ÷ 30 = 3

Molecular formula = (CH₂O)₃ = C₃H₆O₃

Step 4: Check the options

• A. C₄H₁₀O₂ is not a multiple of CH₂O (the C:H ratio is 2:5), so it is wrong.
• B. C₃H₁₀O₂ has the wrong ratio, so it is wrong.
• C. C₃H₆O₃ has the ratio 1:2:1 and a mass of 36 + 6 + 48 = 90, so it is correct.
• D. C₂H₂O₄ has the wrong ratio (its mass is 24 + 2 + 64 = 90, which matches, but the ratio is not CH₂O), so it is wrong.

Option D is a good trap: its mass is 90, but it does not have the empirical formula CH₂O. Always check both the ratio and the mass.

Answer: C

Exam tip: Divide the molecular mass by the empirical formula mass to get n, then multiply every subscript by n. If the options look similar, check that the atom ratio matches the empirical formula as well as the mass.