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 :

Solution

Chemistry ma’am · 3 steps

You already know the molar mass, so turn each percentage straight into grams per mole and count atoms.

1. Oxygen by difference. Oxygen is whatever is left: 100 - 42.1 - 6.4 = 51.5%.

   atoms per molecule = ((% of element) × 342)/(100 × atomic mass)

2. Count each atom. Carbon: 0.421 × 342 = 144 g, and 144 / 12 = 12. Hydrogen: 0.064 × 342 = 21.9 g, which gives about 22 H. Oxygen: 0.515 × 342 = 176 g, and 176 / 16 = 11.

   Option	Molar mass	% C	% H
   (a) C₁₄H₂₀O₁₀	348	48.3	5.7
   (b) C₁₂H₂₂O₁₁	342	42.1	6.4
   (c) C₁₂H₂₀O₁₂	356	40.4	5.6
   (d) C₁₁H₁₈O₁₂	342	38.6	5.3

3. So. Only (b) matches both the molar mass and the composition: the formula is C₁₂H₂₂O₁₁, which is sucrose.

The idea to keep. When the molar mass is given, skip the empirical formula. Percentage times molar mass gives grams of each element in one mole, and dividing by atomic mass gives the atom count directly.

Answer from the NTA final answer key. Solution by PrepWiser.