Find the molecular formula of the hydrocarbon if the mass fraction of carbon is 83.3% q (h2) = 36.

Let’s take the mass of hydrocarbon per 100 g.

Find the mass of carbon in the hydrocarbon.

100 g – 100%,

m (H) – 83.3%.

m = (100 g × 83.3%): 100% = 83.3 g.

Let’s find the mass of hydrogen.

100 g – 83.3 = 16.7 g.

Let’s find the amount of matter of atoms in the compound.

n = m: M.

n (H) = 16.7: 1 = 16.7 mol.

n (C) = 83.3: 12 = 7 mol.

We find the ratio of the amounts of substances.

n (C): n (H) = 16.7: 7 = 1: 2.

Let’s find the molar mass of the hydrocarbon.

D (H2) = 2 × 36 = 72.

The formula of the C5H12 hydrocarbon is pentane.

M (C5H12) = 12 × 5+ 12 = 72 g / mol.