Question

The ${{P}_{4}}{{O}_{10}}$ is not used to dry $N{{H}_{3}}$ gas because:
A. ${{P}_{4}}{{O}_{10}}$ reacts with moisture in $N{{H}_{3}}$
B. ${{P}_{4}}{{O}_{10}}$ is not a drying agent
C. ${{P}_{4}}{{O}_{10}}$ is acidic and is $N{{H}_{3}}$ basic
D. ${{P}_{4}}{{O}_{10}}$ is basic and is $N{{H}_{3}}$ acidic

Answer 1

Hint: ${{P}_{4}}{{O}_{10}}$ is phosphorus pentoxide, or tetraphosphorus decaoxide. Phosphorus oxides are used to obtain oxyacids of phosphorus. Ammonia gas is not dried over dehydrating agents as it will react to form another compound with these agents.

Complete answer: Ammonia gas, $N{{H}_{3}}$ acts as a Lewis base, as the lone pair of electrons on the nitrogen atom of this gas, can easily be donated. Also it liberates hydroxide ions in water, and is weakly alkaline in water.
Due to this basic nature of ammonia, it is dried, so that it does not act as a base. The drying of ammonia is a useful method for the commercial use of this gas. This method uses ammonia gas to react with oxides to lower its basic nature.
 Phosphorus pentoxide, ${{P}_{4}}{{O}_{10}}$ is not a suitable compound to dry ammonia gas. This is because; phosphorus pentoxide is highly reactive in moisture and forms Meta phosphoric acid. The reaction is,
\[{{P}_{4}}{{O}_{10}}+2{{H}_{2}}O\to 4HP{{O}_{3}}\] ,
Further, this, Meta phosphoric acid, reacts with ammonia gas to form a phosphate compound called Meta ammonium phosphate. The reaction is,
\[HP{{O}_{3}}+N{{H}_{3}}\to N{{H}_{4}}P{{O}_{3}}\]
Hence, ${{P}_{4}}{{O}_{10}}$ is not used to dry ammonia as it will react with moisture and will further form Meta ammonium phosphate.

So, option A is correct.

Note: Ammonia gas is prepared through a well known process, called Haber’s process, where hydrogen and nitrogen reacts to form ammonia. In order to dry ammonia gas, oxide of calcium, CaO, called quicklime or calcium oxide, is used. Ammonia passes through freshly prepared quicklime and rapidly dries it.