Question

A gas that cannot be collected over water is
A)$P{{H}_{3}}$
B)$S{{O}_{2}}$
C)${{O}_{3}}$
D)${{N}_{2}}$

Answer 1

Hint: In the process of collection of gas over water a layer of gases is present in equilibrium with that of water.
The gases which are insoluble in water can be collected over water whereas gases which are soluble cannot be collected this way.

Complete answer:
Here in the question four options of different gases are given and we have to select which gas cannot be collected over water during a chemical reaction involving the evolution of a gas.
In the process of gas collection over water, the amount of gas collected is calculated using Dalton’s law in which it is stated that total pressure in the container is equal to the pressure of the gas collected and pressure of water vapour (${{P}_{total}}={{P}_{gas}}+{{P}_{{{H}_{2}}O}}$).From the pressure we could calculate the number of moles of gas by the equation ($PV=nRT$).
The collection of gas over the water is done in the chemical reactions in which the evolved gas is strictly insoluble in water. As in the gas collection over the water, the gas evolved is forming a layer above the water which is in equilibrium with the liquid layer. And the water molecules are displaced inorder to maintain water level to maintain the pressure inside and outside the container. So if we know the atmospheric pressure and the pressure.
So only the insoluble gases can be collected over the water and if we see the options except$S{{O}_{2}}$, all other gases are insoluble in gases.
$S{{O}_{2}}$reacts water to give sulphurous acid (${{H}_{2}}S{{O}_{3}}$).
$S{{O}_{2}}+{{H}_{2}}O\to {{H}_{2}}S{{O}_{3}}$
As $S{{O}_{2}}$dissolves in water easily and is not collected over water.

Therefore the correct option is option (B).

Note:
While solving the question, our approach should not be about how the given gas reacts with the water since almost all gases are solubility at least in a very minimal level.