Question

A compound contains 4.07% hydrogen, 24.27% carbon and 71.65% chlorine. Its molar mass is 98.96g. What are its empirical and molecular formulas?

Answer 1

Let total mass of the molecule = 100g

Mass of C in the molecule = 24.27g

Mass of H in the molecule = 4.07g

Mass of Cl in the molecule =71.65 g

Number of moles of C in molecule = 24.27/ molar mass of carbon = 24.27 /12 = 2.02

Number of moles of H in molecule = 4.07/ molar mass of hydrogen = 4.07 /1 = 4.07

Number of moles of Cl in molecule = 71.65/ molar mass of nitrogen = 71.65 /35.45 =2.02

Ratio of the number of moles of C, H & Cl

i.e.

C:H:Cl = 2.02 : 4.07 : 2.02 = 1:2:1

So the empirical formula will be..

CH₂Cl

Empirical formula mass = 12.01+2+35.45 = 49.5 g

Now

n = Molecular mass/ empirical formula mass = 98.96/49.5 = 2

Molecular formula = n x empirical formula = 2(CH₂Cl) = C₂H₄Cl₂