Find the formula of the hydrocarbon, the mass fraction of hydrogen in which is 15.79%. The relative density in air is 3.93.

Let’s implement the solution:
1. Let us introduce the designation for the hydrocarbon: CxNy;
2. M (CxHy) = D (air) * M (air) = 3.93 * 29 = 113.97 = 114 g / mol;
3. Determine the amount of moles of hydrogen, carbon:
Y (H) = 15.79 / 1 = 15.79 mol;
m (H) = Y * M = 0.1579 * 114 = 17.99 g;
m (C) = 114-17.99 = 96 g;
Y (C) = m / M = 96/12 = 8 mol.
4. Ratio: X: Y = C: H = 8: 15.79 = 1: 2. The simplest formula: CH2;
5. Molecular formula of the hydrocarbon: C8H18 (octane).
6.M (C8H18) = 12 * 8 + 1 * 18 = 114 g / mol.
Answer: the formula has been established – C8H18 (octane).