Question

The relative density of a gas A with respect to another gas B is 2. The vapor density of the gas B is 20, the vapor density of the gas A is:
A. 30
B. 40
C. 50
D. 60

Answer 1

Vapor density is the density of a vapor in relation to that of hydrogen. It may be defined as the mass of a certain volume of a substance divided by mass of the same volume of hydrogen. The relative density is slightly different from the vapor density as the relative density is the density of any gas with respect to the density of any other random gas whereas the vapor density is the density of a gas with respect to only hydrogen gas.

Complete step by step answer:
As per the question, the relative density of the gas A with respect to the gas B is 2. This can be written mathematically as:
$R.D = \dfrac{{{D_A}}}{{{D_B}}} = 2$
This means that the density of gas A is twice the density of the gas B.
The vapor density from the above equation can be calculated as:
${D_A} = 2{D_B}$
In the question, it is given that ${D_B} = 20$
Thus, the vapor density of A will be equal to = ${D_A} = 2 \times {D_B} = 2 \times 20 = 40$

So, the correct answer is Option B .

Note:
The vapor density indicates whether a gas is denser (greater than one) or less dense (less than one) than air. The density has implications for container storage and personnel safety if a container can release a dense gas, its vapor could sink and, if flammable, collect until it is at a concentration sufficient for ignition. Even if not flammable, it could collect in the lower floor or level of a confined space and displace air, possibly presenting an asphyxiation hazard to individuals entering the lower part of that space.