What is the relative molecular mass of a compound which has empirical formula $CH_{2}O$?   [C= 12, H= 1, O= 16]

Analyze the question

You're given the empirical formula CH₂O and asked for the relative molecular mass. The molecular formula is a whole-number multiple of the empirical formula, (CH₂O)ₙ, so the answer depends on which multiple fits.

Concept

• Empirical formula mass of CH₂O = 12 + (2 × 1) + 16 = 30
• Relative molecular mass = n × 30, so it must be a multiple of 30.

Check each option

• A. 42 ÷ 30 = 1.4, not a whole number ✗
• B. 45 ÷ 30 = 1.5, not a whole number ✗
• C. 126 ÷ 30 = 4.2, not a whole number ✗
• D. 180 ÷ 30 = 6, a whole number ✓

So n = 6, and the molecular formula is (CH₂O)₆ = C₆H₁₂O₆. This is glucose, a very common compound in WASSCE chemistry.

Verify: C₆H₁₂O₆ = (6 × 12) + (12 × 1) + (6 × 16) = 72 + 12 + 96 = 180 ✓

Tip: When a question gives only the empirical formula, find the empirical formula mass first, then pick the option that is an exact whole-number multiple of it. Also remember that CH₂O is the empirical formula of glucose, fructose, and other simple sugars, and glucose has a molar mass of 180.