Question

Benzene contains only carbon and hydrogen and has a molar mass of 78.1 g/mol. Analysis shows the compound to be 7.44% hydrogen by mass. Find the empirical and molecular Formula of benzene.

Answer 1

Hydrogen - 7.44%

Carbon - (100-7.44)% = 92.56%

Lets take 100 g of benzene, then we have

Hydrogen - 7.44 g

Carbon - 92.56 g

n - number of moles

n(H) = 7.44g *1 mol/1.0g = 7.44 mol

n(C) = 92.56 g* 1mol/12.0 g ≈ 7.713 mol

n(C) : n(H) = 7.713 mol : 7.44 mol = 1:1

Empirical formula is CH.

M(CH) = (12.0+ 1.00) g/mol = 13.0 g/mol

M (benzene) = 78.1 g/mol

M (benzene)/M(CH)= 78.1 g/mol/13.0 g/mol = 6

So, molecular formula of benzene is C6H6.