Chemistry, Third edition

8 · THE MOLE

Finding molecular formulae

The simplest formula is not always the ‘true’ or molecular formula: it is only the

simplest ratio of the atoms contained in the substance. If, for example, you calculated

the simplest formula of hydrazine (N 2 H 4 ) from its percentage composition by mass

of nitrogen and hydrogen, you would obtain NH 2. In order to establish the molecular

formula, you need the molecular mass as well as the empirical formula.

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Example 8.6

Dichloroethane has a molecular mass of 99 u. Analysis of a

sample shows that it contains 24.3% carbon, 4.1% hydrogen and

71.6% chlorine. What is its molecular formula?

Answer

Calculate the simplest formula:

Element C H Cl

Percentage composition 24.3 4.1 71.6

Divide by molar masses 24.3 4.1 71.6

12 1 35.5

Molar ratio 1 2 1

Therefore, the simplest formula is CH 2 Cl.

The molecular mass of CH 2 Cl 12 (21)35.549.5 u, so it is not the

molecular formula of dichloroethane.

Multiply the number of each atom by two, to maintain the same ratio, and you get

C 2 H 4 Cl 2 which has a molecular mass of

(212)(41)(235.5)99 u

So, the molecular formula of dichloroethane is C 2 H 4 Cl 2.

Determining molecular formulae

(i)Work out the empirical and molecular formulae for a

compound that has a molar mass of

30 g mol^1 and is of percentage composition 80% carbon

and 20% hydrogen.

(ii)Nicotine contains 74.9% carbon, 8.7% hydrogen and

17.3% nitrogen. The compound contains two nitrogen

atoms per molecule. What are the empirical and

molecular formulae of nicotine?

Exercise 8G