Which acid on heating produces phosphine?

Orthophosphorus acid on disproportionation gives $PH_3$ and another oxoacid of phosphorus '$X$'. The basicity of $X$ is

How many isotopes of nitrogen gas are stable? Which are they?

$A$ 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

The total number of compounds having at least one bridging oxo group among the molecules given below is. . . . . . .
$N_2O_3, N_2O_5, P_4O_6, P_4O_7, H_4P_2O_5, H_5P_3O_{10}, H_2S_2O_3, H_2S_2O_5$

What is the highest oxidation state possessed by phosphorus in its oxyacids?