What are the acid anhydrides of $H_3PO_4$ and $HPO_3$ respectively?

The total number of $\sigma$ and $\pi$-bonds in pyrophosphoric acid are respectively

Two oxides of nitrogen,$NO$ and $NO_2$,react together at $253 \ K$ to form a compound of nitrogen $X$. $X$ reacts with water to yield another compound of nitrogen $Y$. The shape of the anion of $Y$ is:

In the modern process,phosphorus is manufactured by:

Heating $P_4O_{10}$ with water yields ..............

Which oxide of nitrogen is obtained on heating ammonium nitrate at $250 \, ^oC$?