Question

Find the gases which are the densest at standard temperature and pressure.
a.Helium
b.Argon
c.Carbon dioxide
d.Xenon
e.Nitrogen

Answer 1

The density of a substance is defined as the ratio of the mass and the volume of the substance. As here we need to find the density of gases, we have to consider the molar mass of gases instead of mass. From the formula for density, we can understand that the density of gases is proportional to molar mass and inversely proportional to the volume of the gas. Hence, the gas with the highest molar mass and lowest volume is the densest.

Complete step by step answer:
The densest gas means the gas with the highest density.
As we know that density of a gas is the ratio of the molar mass of the gas to the volume of the gas.
Let us consider the same volume for the given gases.
Hence, the gas with the highest molar mass will be the densest.
The molar mass of helium is $4gmol^{-1}$
The molar mass of argon is $40gmol^{-1}$
The molar mass of carbon dioxide is $44gmol^{-1}$
The molar mass of xenon is $131gmol^{-1}$
The molar mass of nitrogen is $28gmol^{-1}$
Hence, the molar mass of Xenon is the highest.

Hence, the correct answer is Option $(D)$.

Note:
Here, we deduced that the gas with the highest molar mass was the densest. But this might not be true for every other gaseous element. For example, consider chlorine and argon. The molar mass of argon ( $39.94gmol^{-1}$ ) is more than chlorine ( $35.45gmol^{-1}$ ), but still the density of chlorine ( $3.2gL^{-1}$ ) is higher than argon ( $1.78gL^{-1}$ ). Hence, we cannot find the densest gas with the help of only molar mass. Also, there is no particular trend for the densities, and thus there should be given enough data to derive it.