Question

Which of the following is known as anhydride of nitric acid?
A) ${N_2}{O_3}$
B) ${N_2}{O_4}$
C) ${N_2}{O_5}$
D) ${N_2}O$

Answer 1

Hint: An acid anhydride is that chemical compound which when reacted with water forms acid. So, try and put together the reactions of all the options given above with water and see which one forms nitric acid.

Complete-step- by- step answer:
An acid anhydride or just simply anhydride is that non-metallic oxide which when treated with water leads to the formation of an acidic solution.
We should also know the chemical formula of nitric acid. It is $HN{O_3}$.
We should now begin by writing the reactions of all the 4 compounds given in the options:
${N_2}{O_3}$ or dinitrogen trioxide reacts with water to produce nitrous acid.

                                       \[{N_2}{O_3} + {H_2}O \to 2HN{O_2}\]

So, we can say that ${N_2}{O_3}$ is an anhydride of nitrous acid.
${N_2}{O_4}$ or dinitrogen tetra-oxide reacts with water to produce both nitric acid and nitrous acid.
                                 \[{N_2}{O_4} + {H_2}O \to HN{O_2} + HN{O_3}\]

Since, it gives a mixture of both nitrous and nitric acid it cannot be called anhydride of nitric acid.
${N_2}{O_5}$ or nitrogen penta-oxide is also known as nitric anhydride because when it reacts with water it leads to the formation of nitric acid.

                                    \[{N_2}{O_5} + {H_2}O \to 2HN{O_3}\]

Since, it leads to the formation of nitric acid on reaction with water it is anhydride of nitric acid.
Although we now have our answer but still for a safe side lets see the last option also.
${N_2}O$ or nitrous oxide is quite less reactive and does not form any acidic solution with water. Infact on heating it breaks down exothermically into ${N_2}$ and ${O_2}$.

                               \[{N_2}O\xrightarrow{\Delta }{N_2} + {O_2}\]

So, from the above discussion we now know that ${N_2}{O_5}$ is the anhydride of nitric acid.

Therefore,the correct option is: C) ${N_2}{O_5}$ .

Note: In questions like this we often get confused in the naming and formulas of various compounds. We might get confused in nitric acid and nitrous acid formulas. So, always remember that nitric acid is $HN{O_3}$ and nitrous acid is $HN{O_2}$.