Chemistry - GOC Question with Solution | TestHub

The conjugate bases are $\mathrm{N}{\mathrm{H}}_2^{-}$ and $\mathrm{P}{\mathrm{H}}_2^{-}$, atoms in the same column, so size decides : phosphorus is larger than nitrogen, so the negative charge is spread over a larger volume in $\mathrm{P}{\mathrm{H}}_2^{-}$, which is the more stable base. $\mathrm{P}{\mathrm{H}}_3$ is the stronger acid.