Question

A compound has 95.25g C and 7.75g H. The molecular mass is 52 amu. What is the molecular formula?

Answer 1

Mass of C = 95.25 g

Molar mass of C = 12 g/mol

Moles of C = mass of C/molar mass of C

                  = 95.25 g / 12 g/mol

                  = 7.94 mol

Mass of H = 7.75 g

Molar mass of H = 1 g/mol

Moles of H = mass of H/molar mass of H

                  = 7.75 g / 1 g/mol

                  = 7.75 mol

Mole ratio of C : H in the compound = 7.94 : 7.75

                                                          = 1 : 1

Hence, the empirical formula of the compound = CH

Let us say that the molecular formula of the compound = (CH)_n      (where, n is an integer)

The molecular mass from the above molecular formula = n x (1 x 12 amu + 1 x 1 amu)

                                                                                        = 13n amu

The actual molecular mass = 52 amu

Now,

13n amu = 52 amu

or, n = 4

Hence, the molecular formula = (CH)_n = (CH)₄ = C₄H₄