Sarah finds a compound has 71.65% Cl, 24.27% C and 4.07% H. The molar mass of this compound is known to be 98.96 g.

Question

a) Determine how many grams of each element are present in 100.0 g of this compound.

b) Determine the number of moles of each element present in 100.0 g of this compound.

c) Determine the empirical formula of this compound. 

d) Calculate the empirical formula mass of this compound.

e) Determine the molecular formula of this compound. 

Answer 1

a)

Cl: 0.7165 x 100.0 g = 71.65 g Cl

C: 0.2427 x 100.0 g = 24.27 g C

H: 0.0407 x 100.0 g = 4.07 g H

b)

Cl: 71.65 g Cl x 1 mol Cl / 35.45 g = 2.021 mols Cl

C: 24.27 g C x 1 mol C / 12.01 g = 2.021 mols C

H: 4.07 g H x 1 mol H / 1.008 g = 4.038 mols H

c) Divide all by the lowest value of mols in order to try to get whole numbers...

Cl = 2.021 / 2.021 = 1 mol Cl

C = 2.021 / 2.021 = 1 mol C

H = 4.038 / 2.021 = 2 mols H

Empirical formula = CH₂Cl

d) 1x12.01 + 2x1.008 + 1x35.45 = 12.01 + 2.016 + 35.45 = 49.48 g/mol = formula mass

e) 98.96 g/mol ÷ 49.48 = 2

Molecular formula = C₂H₄Cl₂