0.01 mole of an organic compound (X) containing 10 % hydrogen, on complete combustion produced 0.9 g H₂ O. Molar mass of (X) is _________ g mol⁻¹.

Solution

Chemistry ma’am · 4 steps

Track the hydrogen: every H atom in the water came from X, so find H per mole of X first.

1. Hydrogen in the water. Moles of water = 0.9/18 = 0.05 mol. Each H₂O carries 2 H atoms, so H atoms = 2 × 0.05 = 0.1 mol, which is 0.1 g of hydrogen.
2. Per mole of X. All of this hydrogen came from 0.01 mol of X. So one mole of X contains 0.1/0.01 = 10 g of hydrogen.
3. Use the percentage. Hydrogen is 10% of the molar mass M of X. That gives the key equation below.

   10/100 × M = 10 g ⇒ M = 100 g mol⁻¹

4. So. The molar mass of X is 100 g mol⁻¹.

The common slip: 50. Taking moles of H equal to moles of water (0.05) misses the 2 H atoms in each H₂O and gives 50, not 100.

The idea to keep. In combustion questions, atoms are conserved. Find the moles of the element in the product, divide by moles of the compound, and match it to the given percentage.

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