Question

Of the following, the anhydride of nitric acid is:
a) \[{{N}_{2}}O\]
b) NO
c) \[N{{O}_{2}}\]
d) \[{{N}_{2}}{{O}_{5}}\]

Answer 1

Hint: Anhydrides of inorganic acids are usually oxides of the non-metallic elements whereas the anhydrides of organic acids, like the acids themselves, mostly contain the carbonyl group in them.

Complete step by step solution:

* Anhydride is any chemical compound obtained by the elimination of water from another compound.
* There are different oxides of nitrogen, they are a mixture of gases that are composed of nitrogen and oxygen. Two of the most toxic compounds are nitric oxide (NO) and nitrogen dioxide (\[N{{O}_{2}}\]). Other gases that belongs to this group are nitrogen monoxide, nitrous oxide (\[{{N}_{2}}O\]), dinitrogen trioxide (\[{{N}_{2}}{{O}_{3}}\]) and dinitrogen pentoxide (\[{{N}_{2}}{{O}_{5}}\])
* Two of its oxides are acid anhydrides, that is, they react with water to form nitrogen-containing oxyacids. Dinitrogen trioxide (\[{{N}_{2}}{{O}_{3}}\]) is the anhydride of nitrous acid,
(\[HN{{O}_{2}}\]), and dinitrogen pentoxide(\[{{N}_{2}}{{O}_{5}}\]) is the anhydride of nitric acid (\[HN{{O}_{3}}\]).
\[{{N}_{2}}{{O}_{5}}\] is an acidic oxide which reacts with water with hissing sound, forming nitric acid
\[{{N}_{2}}{{O}_{5}}+{{H}_{2}}O\to 2HN{{O}_{3}}\]
Thus, it is known as nitric anhydride and the correct option is (d).

Additional Information:

1. In symmetrical acid anhydrides, where only one constituent carboxylic acid is used to form the compound, only the prefix of the original carboxylic acid is used and the suffix "anhydride" is added
2. In asymmetrical acid anhydrides, where two different carboxylic acids were used to give the anhydride the prefixes from both acids reacted are listed before the suffix.

Note: If an anhydride produces base on addition of water to it, then the anhydride will be anhydride of base. These are usually oxides of metal. (Exception: Some higher valent metallic oxides can form acids when treated with water. They should be treated as anhydride of acid)