The substance contains carbon and hydrogen, mass fractions of which are respectively equal to 75% and 25%.

The substance contains carbon and hydrogen, mass fractions of which are respectively equal to 75% and 25%. The hydrogen vapor density of the substance is 8. Determine the formula of this organic compound.

Given:
ω (C) = 75%
ω (H) = 25%
D H2 (CxHy) = 8

To find:
CxHy -?

Decision:
1) Let m (CxHy) = 100 g;
2) m (C) = ω (C) * m (CxHy) / 100% = 75% * 100/100% = 75 g;
3) n (C) = m (C) / M (C) = 75/12 = 6.25 mol;
4) m (H) = ω (H) * m (CxHy) / 100% = 25% * 100/100% = 25 g;
5) n (H) = m (H) / M (H) = 25/1 = 25 mol;
6) x: y = n (C): n (H) = 6.25: 25 = 1: 4;
7) M (CH4) = Mr (CH4) = Ar (C) * N (C) + Ar (H) * N (H) = 12 * 1 + 1 * 4 = 16 g / mol;
8) M (CxHy) = D H2 (CxHy) * M (H2) = 8 * 2 = 16 g / mol.

Answer: Unknown substance – CH4.