Question

Atomic mass of chlorine is 35.5. What is its vapor density?

Answer 1

Hint:The vapor density indicates whether a gas is denser or less dense than other gas/air. If we calculate vapor density with respect to air then it would tell us that the particular gas will rise in air or sink, depending upon the value whether it is less than 1 or more.

Complete answer:
Vapor density is the density of a gas/vapor in relation to that of air, oxygen and hydrogen.
By taking hydrogen as the reference substance, the vapor density may be defined as mass of a certain volume of a substance divided by mass of same volume of hydrogen under identical conditions of pressure and temperature
Vapor density= mass of n molecules of gas/ mass of n molecules of hydrogen.
Vapor density= molar mass of gas/molar mass of ${H_2}$
Vapor density= $\dfrac{1}{2} \times molar \,mass $
Or we can say by taking the density of hydrogen as 1, this ratio is equal to the half the relative molecular mass of the gas.
Here, given that, the molar mass of chlorine is 35.5.
Chlorine is present in a gaseous state as $Cl_2$.
Therefore the molar mass of chlorine gas is 71.

Hence the vapor density of chlorine gas will be $\dfrac{{71}}{2} = 35.5$

Note:
Vapor density is unit less quantity as it is the ratio of molar mass of two gases.
Vapor density with respect to air: This is the vapor density of substance with respect to air so air is given the vapor density of 1. For calculation air has given the molecular weight of 28.97 atomic mass units. And all the other gas and vapor molecular weights are divided by this number to derive their vapor density with respect to air.