Question

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

Answer 1

Hint: The answer to this question is a gas that is used in the contact process. The gas that will react with water cannot be collected over water.

Complete step by step answer:
Insoluble gases don’t dissolve in water and thus can be collected over it. For example, Nitrogen, Oxygen, Carbon dioxide, etc. Soluble gases dissolve easily in water and thus can’t be separated from it. Here, Sulphur dioxide reacts with water to give sulphurous acid. The reaction sulphur dioxide with water is:
\[S{O_2} + {H_2}O \to {H_2}S{O_3}\]
So, sulphur dioxide cannot be collected over water. Therefore, the correct option is (b).

Additional Information:
* The first option is phosphine (${\text{P}}{{\text{H}}_{\text{3}}}$). Phosphine is a colourless, flammable, toxic gas and is classified as pnictogen hydride. Pure phosphine odourless, but technical grade samples have a highly unpleasant odour like garlic or rotting fish, due to the presence of substituted phosphine and diphosphane.
* The third option is ozone. Ozone is an inorganic pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope dioxygen. Ozone is formed from dioxygen by the action of ultraviolet light and electrical discharges within the Earth’s atmosphere.
* The final option is dinitrogen gas. Nitrogen is the most abundant element in the atmospheric air than oxygen. Dinitrogen forms about 78% of Earth’s atmosphere, making it the most abundant uncombined element.

Note: The approach for this question is not through the solubility comparison of the gases in water. All the gases are soluble in water. Here, the reaction of that gas with the water molecule is more important.