The hydrocarbon composition is C = 85.7%, H = 14.3%, D air = 0.9655. Find the molecular formula of the substance.

Given:
Unknown substance – CxHy
D air. (CxHy) = 0.9655
ω (C in CxHy) = 85.7%
ω (H in CxHy) = 14.3%

To find:
CxHy -?

Decision:
1) M (CxHy) = D air. (CxHy) * M (air) = 0.9655 * 29 = 28 g / mol;
2) Mr (CxHy) = M (CxHy) = 28;
3) N (C in CxHy) = (ω (C in CxHy) * Mr (CxHy)) / (Ar (C) * 100%) = (85.7% * 28) / (12 * 100%) = 2 ;
4) N (H in CxHy) = (ω (H in CxHy) * Mr (CxHy)) / (Ar (H) * 100%) = (14.3% * 28) / (1 * 100%) = 4 ;
5) Unknown substance – C2H4 – ethene.

Answer: Unknown substance – C2H4 – ethene.