Question

Ammonia can be dried by:
A. Conc. ${ H }_{ 2 }{ SO }_{ 4 }$
B. ${ P }_{ 4 }{ O }_{ 10 }$
C. CaO
D. Anhydrous ${ CaCl }_{ 2 }$

Answer 1

Hint: To choose a drying agent, it is important that we find the one which will not react with the compound that we are trying to dry.

Complete step by step answer:

We will look at all the options provided one by one to reach the correct answer.

Conc. ${ H }_{ 2 }{ SO }_{ 4 }$ : We know ${ NH }_{ 3 }$ is a base and concentrated ${ H }_{ 2 }{ SO }_{ 4 }$ (which is used for drying some gases) is a very strong acid and oxidant. We should think the danger of trying to bubble ammonia gas through concentrated sulphuric acid would be obvious. Ammonia gas dissolves extremely rapidly in water to produce the fountain effect, so with conc. sulphuric acid we can expect something that would be broader than that.
$2{ NH }_{ 3 } + { H }_{ 2 }{ SO }_{ 4 } \rightarrow { ({ NH }_{ 4 }) }_{ 2 }{ SO }_{ 4 }$

${ P }_{ 4 }{ O }_{ 10 }$: It is acidic oxide, so if the ammonia is damp (why else would you be drying it), then you would have either phosphorus or phosphoric acid which would react with and contaminate your ammonia.
${ P }_{ 4 }{ O }_{ 10 } + 12{ NH }_{ 3 } \rightarrow 4[({ NH }_{ 4 })_{ 3 }{ PO }_{ 4 }] + 6{ H }_{ 2 }O$

CaO: Calcium oxide is used as a drying agent for ammonia as both are basic in nature and do not undergo any reaction, while any acidic drying agent undergoes reaction with ammonia.
Anhydrous ${ CaCl }_{ 2 }$: Ammonia gas forms an additional compound with anhydrous calcium chloride. Hence it cannot be used as a drying agent.
$8{ NH }_{ 3 } + { CaCl }_{ 2 } \rightarrow { CaCl }_{ 2 }.8{ NH }_{ 3 }$ (Additional compound )

From all these statements we can conclude that option C is the correct answer.

Note: The chemical formula of lime or quicklime is CaO. The chemical name of lime is calcium oxide. On the other hand, the chemical formula of lime water is ${ Ca(OH) }_{ 2 }$ and the chemical name of this substance is calcium hydroxide.