Establish the molecular formula of a hydrocarbon if it is known that it contains 80% carbon

Establish the molecular formula of a hydrocarbon if it is known that it contains 80% carbon, 20% hydrogen, and the relative density of vapor in air is 1.034.

Given:
CxHy
ω (C in CxHy) = 80%
ω (H in CxHy) = 20%
D air. (CxHy) = 1.034

To find:
CxHy -?

Decision:
1) Calculate the molar mass of CxHy:
M (CxHy) = D air. (CxHy) * M (air) = 1.034 * 29 = 30 g / mol;
2) Determine the relative molecular weight of CxHy:
Mr (CxHy) = M (CxHy) = 30;
3) Calculate the index of C atoms in CxHy:
N (C in CxHy) = (ω (C in CxHy) * Mr (CxHy)) / (Ar (C) * 100%) = (80% * 30) / (12 * 100%) = 2;
4) Calculate the index of H atoms in CxHy:
N (H in CxHy) = (ω (H in CxHy) * Mr (CxHy)) / (Ar (H) * 100%) = (20% * 30) / (1 * 100%) = 6;
Unknown substance – C2H6.

Answer: Unknown substance -C2H6.