What is the molecular formula of pyrophosphoric acid?

Which of the following oxides is least acidic?

The purest form of $N_2$ is prepared by the thermal decomposition of:

The correct order of boiling points of hydrides of $15^{th}$ group elements is

The decreasing order of the number of ionisable hydrogen atoms in the following molecules is:
$I$. $H_3PO_4$
$II$. $H_3PO_3$
$III$. $H_3PO_2$
$IV$. $H_4P_2O_6$

Which of the following reactions give phosphine $(PH_3)$?
$(i)$ Reaction of calcium phosphide with water
$(ii)$ Heating white phosphorus with concentrated $NaOH$ solution in an inert atmosphere
$(iii)$ Heating red phosphorus with alkali