Question

Vapour density of gas is 11.2. Volume occupied by 2.4g of this at STP will be:
A. 11.2L
B. 2.24L
C. 22.4L
D. 2.4L

Answer 1

Hint: Vapour density is not a constant value. It changes with the change in the temperature, pressure and volume of a gas. It is related to the molar mass of a substance. So, we can find the vapour density of a gas at certain conditions by knowing the molar mass of the gas.

Complete step by step solution:
-Vapour density is the density of a gas relative to hydrogen gas. It is measured by comparing the density of a gas with that of hydrogen gas at the same temperature and pressure. It refers to the relative weight of a certain gas.
-Gases are also a state of matter and so they also have weight and volume though they do not have any definite shape and a definite volume. This is because the atoms in gases are loosely held together by one another because of low attractive forces called vander-waal forces.
-Vapour density is related to the molar mass of a gas and can be written as
Molar mass = 2 x Vapour density
Molar mass of a gas is the mass of 1 mole of the gas particles. It is found by multiplying the mass of a single atom of that particular gas with the avogadro’s number because it represents the number of particles in 1 mole of a substance.
-In this question, we are given that the mass of gas = 2.4 g
Molar volume for any gas will be 22.4 L at STP. The molar mass of the gases varies but not the molar volume.
-We can find the moles of the gas as the vapour density has been given to us. As we know
Molar mass = 2 x Vapour density
So, the molar mass will be equal to 2x11.2=22.4 g
Now, moles = $\dfrac{\text{wt}\text{. in grams}}{molecular\text{ wt}\text{.}}=\dfrac{2.4}{22.4}$
-We know that volume for 1 mole of a gas is always equal to 22.4L. So, volume for n moles of a gas will be 22.4 x n.
So the volume of the given gas will be 22.4 x $\dfrac{2.4}{22.4}$= 2.4L.

Therefore the correct option is D. 2.4L.

Note: The vapour pressure of air is considered to be equal to 1. If any gas has the vapour pressure of more than 1, it is said to be lighter than air and so it sinks in air. If the vapour pressure of gas is less than 1, it is said to float in air or rise in air.