Question

During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to

• A. Absorb the evolved gas
• B. Moisten the gas
• C. Absorb moisture from the gas
• D. Absorb Cl^- ions from the evolved gas

Answer 1

The correct option is C

Absorb moisture from the gas

The explanation for the Correct answer

(C)

• Anhydrous or fused calcium chloride is highly hygroscopic in nature it readily absorbs moisture, also it is deliquescent in nature, and is therefore used as a drying agent in a desiccator.
• Calcium is a good dehydrating agent. It has the property to absorb moisture. Hence it is used as a desiccant to dry gases and Hydrocarbons in industries.

The chemical reaction for absorption of

$\underset{\mathrm{calcium}\mathrm{chloride}}{{\mathrm{CaCl}}_2\left(\mathrm{s}\right)}+\underset{\mathrm{water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)}\to \underset{\mathrm{calcium}\mathrm{oxide}}{\mathrm{CaO}\left(\mathrm{s}\right)}+\underset{\mathrm{hydrogen}\mathrm{chloride}}{\mathrm{HCl}\left(\mathrm{g}\right)}$

Therefore during the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to absorb moisture from the gas.

The explanation for the incorrect option:

(A) ${\mathrm{CaCl}}_2$ is a dehydrating agent so it can absorb the moisture from the gas and smoothen the preparation of hydrochloride gas. It can't absorb the gas evolved from the reaction.

(B) ${\mathrm{CaCl}}_2$ is a dehydrating agent so it can't moisten the gas.

(D) ${\mathrm{CaCl}}_2$ is not able to absorb ${\mathrm{Cl}}^{-}$ions from the evolved gas. So, these are, therefore, incorrect options.
Hence, the correct option is (C).