The mass fraction of carbon in the hydrocarbon is 85.7%, and the relative density of the hydrocarbon in air is 3.31.

The mass fraction of carbon in the hydrocarbon is 85.7%, and the relative density of the hydrocarbon in air is 3.31. Determine the formula of the substance

w (H) = 100% -85.7% = 14.3%
Let m (connection) = 100 g
then
m (H) = 14.3 g n (H) = 14.3 \ 1 = 14.3 mol
m (C) = 85.7 g n (C) = 85.7 \ 12 = 7.14 mol
n (H): n (C) = 14.3: 7.14 = 2: 1
The simplest formula is CH2 (14 g / mol)
M = 3.31 * 29 (M air) = 95.99 g / mol
95.99 \ 14 = 7
We multiply the number of carbon and hydrogen atoms by 7 in the simplest formula, we get: С7H14
Answer: C7H14