Answer

$\mathrm{H_3} \mathrm{PO_4}$

Since there are three OH groups present in $\mathrm{H_3} \mathrm{PO_4}$, its basicity is three i.e., it is a tribasic acid.

Answer

$\mathrm{H_3} \mathrm{PO_3}$, on heating, undergoes disproportionation reaction to form $\mathrm{PH_3}$ and $\mathrm{H_3} \mathrm{PO_4}$. The oxidation numbers of $\mathrm{P}$ in $\mathrm{H_3} \mathrm{PO_3}, \mathrm{PH_3}$, and $\mathrm{H_3} \mathrm{PO_4}$ are $+3,-3$, and +5 respectively. As the oxidation number of the same element is decreasing and increasing during a particular reaction, the reaction is a disproportionation reaction.

$$ \underset{\substack{\text{Orthophosphorous acid} \\ (+3)}}{4 \mathrm{H_3} \mathrm{PO_3}} \xrightarrow{\Delta} \quad \underset{\substack{\text{Orthophosphoric acid} \\ (+5)}}{3 \mathrm{H_3} \mathrm{PO_4}} \quad+\quad \underset{\substack{\text{Phospine} \\ (-3)}}{\mathrm{PH_3}} $$