How many $H-$ atoms are directly bonded to the phosphorus atom in hypophosphorous acid?

What compounds are formed when phosphine $(PH_3)$ is absorbed in a solution of mercuric chloride $(HgCl_2)$?

$A$ obtained by Ostwald's method involving air oxidation of $NH_3$,upon further air oxidation produces $B$. $B$ on hydration forms an oxoacid of Nitrogen along with evolution of $A$. The oxoacid also produces $A$ and gives positive brown ring test.

Which of the nitrogen oxides is obtained when ammonium nitrate is heated?

Which of the following compounds is used as a refrigerant?

$A$ tetra-atomic molecule $(A)$ on reaction with nitrogen $(I)$ oxide,produces two substances $(B)$ and $(C).$ $(B)$ is a dehydrating agent in its monomeric form while substance $(C)$ is a diatomic gas which shows almost inert behaviour. The substances $(A)$,$(B)$,and $(C)$ respectively will be