Question

At N.T.P. the density of air is 0.001293 g $m{L}^{-1}$ Its vapour density is :

• A. 143
• B. 14.3
• C. 1.43
• D. 0.143

Answer 1

Answer: (B)

14.3

We know that: Molecular Mass= 2x Vapour Density

At STP or under standard conditions of pressure and temperature, 22.4 l of any gas equals to 1 mole.

That is, 1 mole = 22.4 l

The density of air is given as 0.001293g/ml

The volume of gas=22400 ml

We know that mass= density x volume

Therefore, rearranging the above equation, Mass= 0.001293 x 22400= 28.96 g

Also we know that, Molecular Mass= 2 x Vapour Density

Rearranging the equation we get,

Vapour Density= Molecular Mass ÷ 2

=28.96÷2=14.48 gm

Hence, the correct option is $\text{B}$