The mass fraction of carbon in the hydrocarbon is 90.57%. Determine its molecular formula.

1. Find the masses and quantities of elements in 100 g of hydrocarbon:

m (C) = (m (in-va) * ω (C)) / 100% = (100 g * 90.57%) / 100% = 90.57 g.

n (C) = m (C) / M (C) = 90.57 g / 12 g / mol = 7.55 mol.

ω (H) = 100% – ω (C) = 100% – 90.57% = 9.43%.

m (H) = (m (in-va) * ω (H)) / 100% = (100 g * 9.43%) / 100% = 9.43 g.

n (H) = m (H) / M (H) = 9.43 g / 1 g / mol = 9.43 mol.

2. Let’s find the formula of the substance:

n (C): n (H) = 7.55 mol: 9.43 mol = 1: 1.25 = 4: 5 ⇒ C4H5 – a substance with such a formula does not exist, double it ⇒ C8H10 – such a substance exists.

Answer: C8H10 -ethylbenzene.