Chemistry Mole Question

Question

In 1991, replacement compounds for Freons went into production. One of them, HFC -134a, contains 23.53% C,1.96% H, and 74.51% F(by mass). If 1.0L of HFC-134a gas at STP has a mass of 4.55g, what is the molecular formula of HFC-134a?

Answer 1

At STP 1 mole of gas = 22.4 L

1 L x 1 mol/22.4 L = 0.0446 moles

molar mass = 4.55 g / 0.0446 moles = 102 g/mol

mol C = 23.53 g C x 1 mol/12 g = 1.96 mol C

mol H = 1.96 g H x 1 mol/1 g = 1.96 mol H

mol F = 74.51 g F x 1 mol/19 g = 3.92 mol F

mole ratio of elements = 1.96 : 1.96 : 3.92 = 1 : 1 : 2

Empirical formula = CHF₂ and molar mass = 12 + 1 + 38 = 51 g/mol

To find molecular formula, find how many empiricals it takes to make 1 molecular:

102 g/mol ÷ 51 g/mol = 2

Molecular formula = C₂H₂F₄