The mass fraction of carbon in the alkane is 83.3%. The relative density of the vapors of this hydrocarbon

The mass fraction of carbon in the alkane is 83.3%. The relative density of the vapors of this hydrocarbon in air is 2.48. Find the molecular formula of a substance.

Given: CxNu – alkane (saturated hydrocarbon)
W (C) = 83.3%

D (air) = 2.48;

M (air) = 29

Find: formula.

M (CxHy) = D (air) * M (air) = 2.48 * 29 = 71.92 = 72 g / mol.
Let’s determine the amount of carbon:
Y (C) = 83.3 / 12 = 6.9 = 7 mol

CnH2n + 2 is the general formula for alkanes.
CnH2n + 2 = 72

12n + 2n + 2 = 72

14n = 70

n = 5

Let’s establish the formula: C5H12 – pentane.

M (C5H12) = 72 g / mol.

Answer: molecular formula C5H12 (pentane).