Question

(a) Calculate the empirical formula of the compound having 37.6% sodium, 23.1% silicon and 39.3% oxygen (O = 16; Na = 23; Si = 28).
(b) The empirical formula of a compound is C₂H₅. It has a vapour density of 29. Determine the relative molecular mass of the compound and hence its molecular formula.

Answer 1

(a)

Element	Atomic weight	Percentage	Relative number of moles	Simplest ratio	Whole number ratio
Na	23	37.6	$\frac{37.6}{23}=1.63$	$\frac{1.63}{0.82}=1.99$	2
Si	28	23.1	$\frac{23.1}{28}=0.82$	$\frac{0.82}{0.82}=1$	1
O	16	39.3	$\frac{39.3}{16}=2.46$	$\frac{2.45}{0.82}=2.99$	3

Therefore, the empirical formula of compound is Na₂SiO₃.

(b) It is given that empirical formula of a compound is C₂H₅.
Empirical formula mass of the compound = [(12 $\times$ 2) + (1 $\times$ 5)] g = 29 g
As we know,
Molecular mass of a gas = Vapour density $\times$ 2
= (29 $\times$ 2) g = 58 g
Molecular formula of the compound = (Empirical formula)_n
$n=\frac{\mathrm{Molecular}\mathrm{formula}\mathrm{mass}}{\mathrm{Empirical}\mathrm{formula}\mathrm{mass}}=\frac{58}{29}=2$
Therefore, molecular formula is C₄H₁₀.