Number of hydrogen atoms per molecule of a hydrocarbon A having 85.8% carbon is __________
(Given : Molar mass of A = 84 g mol⁻¹)

Solution

Chemistry ma’am · 4 steps

Turn the percentage into grams of carbon in one mole, and the rest of the 84 g has to be hydrogen.

1. Carbon in one mole. One mole of A weighs 84 g, and 85.8% of that is carbon: 0.858 × 84 = 72.1 g. Dividing by 12 g per mole of C gives about 6, so there are 6 C atoms per molecule.
2. Hydrogen is the rest. A hydrocarbon has only C and H. With exactly 6 C atoms the hydrogen mass is 84 - 6 × 12 = 12 g, which is 12 mol of H atoms.

   C₆H₁₂: 6 × 12 + 12 × 1 = 84 g mol⁻¹

3. Cross-check. The mole ratio is 85.8/12 : 14.2/1 = 7.15 : 14.2 = 1 : 2, so the empirical formula is CH₂ with mass 14. Then n = 84/14 = 6, which gives C₆H₁₂ again.
4. So. A is C₆H₁₂, so each molecule has 12 hydrogen atoms.

The common slip: 2. Stopping at the empirical formula CH₂ gives 2, not 12. Scale it up by n = 84/14 = 6.

The idea to keep. When the molar mass is given, multiply each percentage by it to get grams of each element per mole. Then divide by atomic mass to count atoms directly.

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