Question

The gas that cannot be collected over water is:
(a) \[{N_2}\]
(b) \[{O_2}\]
(c) \[S{O_2}\]
(d) \[P{H_3}\]

Answer 1

Hint: A gas which reacts with water cannot be collected over it. The gas reacts with water to form a whole different compound.

Complete step by step answer:
- To collect a gas over water, it is easy to determine its volume. It is done by measuring the height of displaced water which is accomplished by placing a tube into an inverted bottle.
- The opening of the inverted bottle is immersed in a larger container of water. The gas collected over water is not pure but is mixed with vapours from the evaporation of water.
- Dalton's law can be used to calculate the amount of the desired gas by subtracting the contribution of the water vapor. The gases which do not react with water or are insoluble in water are collected over water.
- From the options above discussion we can say that the only gas which can’t be collected over water is , as it reacts with water to form \[{H_2}S{O_3}\], which is a weak acid. The chemical reaction demonstrating the change is:
\[S{O_2} + {H_2}O \to {H_2}S{O_3}\]
Other given gases do not react with water, hence can be easily collected over water.
Hence, the correct answer is (C) \[S{O_2}\].

Note: Remember that oxygen has a low solubility in water, despite the fact that aquatic life depends on it. The solubility of oxygen is only 264 μM at $25^oC$.