Question

The anhydride of nitrous acid is
a) $ {{N}_{2}}{{O}_{3}} $
b) NO
c) $ {{N}_{2}}O $
d) $ {{N}_{2}}{{O}_{4}} $

Answer 1

Hint: Water molecules contained inside crystals are known as water(s) of crystallisation or water(s) of hydration in chemistry. In the production of crystals from aqueous solutions, water is frequently present. Many chemicals absorb water molecules into their crystalline frameworks after crystallisation from water or water-containing solvents. Although a sample's water of crystallisation may usually be eliminated by heating it, the crystalline characteristics are frequently lost. The dihydrate of sodium chloride, for example, is unstable at ambient temperature.

Complete answer:
Nitrous acid (molecular formula $ HN{{O}_{2}} $ ) is a weak, monoprotic acid that can only be found in solution, in the gas phase, and as nitrite ( $ N{{O}_{2}} $ ) salts. To produce diazonium salts from amines, nitrous acid is needed. The resultant diazonium salts are used as reagents in azo dye coupling processes.
A dehydration reaction, also known as Zimmer's Hydrogenesis in chemistry, is a conversion in which the reacting molecule or ion loses water. Dehydration reactions, on the other hand, are typical processes that are the inverse of hydration reactions. Sulfuric acid and alumina are two common dehydration agents used in organic synthesis. Heating frequently affects dehydration responses.
Dinitrogen trioxide $ \left(\mathrm{N}_{2} \mathrm{O}_{3}\right. $ is the anhydride of nitrous acid, $ \mathrm{HNO}_{2} $ .
 $ 2 \mathrm{HNO}_{2} \rightarrow \mathrm{N}_{2} \mathrm{O}_{3}+\mathrm{H}_{2} \mathrm{O} *(\text { Dehydration). } $
Hence option A is correct.

Note:
The ozone budget of the lower atmosphere, the troposphere, is influenced by nitrous acid. Nitrous acid is produced by the heterogeneous interaction of nitric oxide (NO) with water. The result of this reaction rapidly photolysis to hydroxyl radicals when it occurs on the surface of atmospheric aerosols.