The mass fraction of chlorine in phosphorus chloride is 77.5%. Determine the simplest formula of chloride.

1. Let’s find the masses and quantities of elements in 100 g of hydrocarbon:

m (Cl) = (m (in-va) * ω (Cl)) / 100% = (100 g * 77.5%) / 100% = 77.5 g.

n (Cl) = m (Cl) / M (Cl) = 77.5 g / 35.5 g / mol = 2.18 mol.

ω (P) = 100% – ω (Cl) = 100% – 77.5% = 22.5%.

m (P) = (m (in-va) * ω (P)) / 100% = (100 g * 22.5%) / 100% = 22.5 g.

n (P) = m (P) / M (P) = 22.5 g / 31 g / mol = 0.73 mol.

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

n (P): n (Cl) = 0.73 mol: 2.18 mol = 1: 3 ⇒ PCl3.

Answer: PCl3.