Which of the following oxides of nitrogen is the anhydride of $HNO_3$?

On heating,compound $(A)$ gives a gas $(B)$ which is a constituent of air. This gas when treated with $3 \, \text{moles}$ of hydrogen $(H_2)$ in the presence of a catalyst gives another gas $(C)$ which is basic in nature. Gas $(C)$ on further oxidation in moist condition gives a compound $(D)$ which is a part of acid rain. Identify compounds $(A)$ to $(D)$ and also give necessary equations of all the steps involved.

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

What are $X$ and $Y$, respectively in the following reactions?
$Au + \text{aqua regia} \longrightarrow AuCl_4^{-} + H_2O + X$
$Pt + \text{aqua regia} \longrightarrow PtCl_6^{2-} + H_2O + Y$

Which of the following ions does not exist in an aqueous solution?

What is the $P-P-P$ bond angle in white phosphorus?