Question

The density of air is 0.001293$gm{l^{ - 1}}$. Its vapour density is:
A.148
B.14.48
C.1.448
D.0.1448

Answer 1

Hint: Relative molecular mass or molecular weight is the mass of the molecule. It is calculated as the sum of the atomic weights of each constituent element multiplied by the number of atoms of that element in the molecular formula.
 The relation between vapour density and molecular mass is
 Molecular mass= 2$ \times $vapour density.

Complete step by step answer:
We know that,
Molecular mass= 2$ \times $vapour density
But we are given only density in the question
Density= 0.001293 g/ml
Density=$\dfrac{{mass}}{{volume}}$
As the density is 0.001293 g/ml,
We can say that 0.001293 grams of air is dissolved in 1 millilitre.
This means 1 ml has 0.001293 gm.
We know that 1 mole of any substance is contained in 22.4 litre
We are given 1 ml contains 0.001293 gram
1 ml contains 0.001293 gram
$1 \times {10^3}$(1 litre) contains 1.293 gm
22.4$ \times 1$ litre contains (1.293$ \times $22.4) gms
22.4 litre contains 28.96 gms
So we know that the weight at 22.4 litre is considered to be the molecular weight.
Molecular weight= 28.96 grams
From the relation of vapour density and molecular weight we can find the vapour density of air
Vapour density= $\dfrac{{Molecular{\text{ weight}}}}{2}$
                           =$\dfrac{{28.96}}{2} = 14.48$
So the answer is (B) i.e. 14.48

Note:
 Vapour density is the density of a particular gas or vapour relative to that of hydrogen at the same pressure and temperature.Vapour density is the density of a vapour in relation to that of hydrogen. It may be defined as mass of a certain volume of a substance divided by the mass of the same volume of hydrogen. vapour density = mass of n molecules of gas / mass of n molecules of hydrogen.