Determination of the Molecular Formula of a Compound

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• The empirical formula of the compound is written.

• The empirical formula mass is calculated by adding the atomic masses of all atoms present in the empirical formula.

• The molecular mass or molar mass is obtained from experiment or from the vapour density relationship:

molecular mass = 2 x vapour density

• Obtain the molecular formula from: Molecular formula = n x empirical formula

Percentage Composition of a Compound

The mass percentage of each constituent element present in any compound is called its percentage composition. This may be obtained from the molecular formula of the compound by using the relationship:

This can also be determined by a suitable chemical method of chemical analysis of the compound by using the same relationship.

Problems

10. Determine the percentage composition of potassium nitrate, (KNO₃) (Atomic masses in amu are, K = 39, N= 14, O=16)

Solution

Formula mass of KNO₃ = (39 amu + 14 amu + 3 x 16 amu) = 101 amu

Then,

The percentage composition of KNO₃ is K=38.6%, N=13.9%, O=47.5%.

11. A compound contains 75% carbon and 25% hydrogen. Determine its empirical formula. The molecular mass of this compound is 16 amu. Determine its molecular formula also. The atomic masses are: C = 12 amu, H = 1 amu.

Solution

The above results are written as follows:

So, The empirical formulae of the compound = C₁H₄ or CH₄

The empirical formula mass = (1 x 12) + (4 x 1) = 12 + 4 = 16 amu

Molecular mass (given) = 16 amu

Therefore, Molecular formula = 1 x Empirical formula = 1 x CH₄ = CH₄.