Statement (I) : p^K_a of NH₃ > p^K_a of PH₃

Statement (II) : N-H bond is less polar than P-H bond

Acidic strength in Hydrides of any group is explained on the basis of X-H bond enthalpy but not on the basis of X-H bond polarity.

EN of H(2.1) ;  EN of N(3.0) ; EN of P(2.5)

N - H bond is more polar than P-H bond

N - H bond enthalpy is more than P - H bond enthalpy

$\therefore$NH₃ is a weaker acid (lesser K_a & higher p^K_a   as compared to PH₃)