As₂O₃< SiO₂< P₂O₃< SO₂ Acid strength

ACIDIC STRENGTH: acidic strength cab be described as the tendency of an acid to give proton to the solution.

Acidic strength of oxides decreases down the group and increases along period from left to right.

H₂O > H₂S > H₂Se > H₂Te in order of Thermal stability.

As the size of the bond length increases, more easily it can be dissociated and become less stable. Down the group atomic size increases and thermal stability decreases.

(ii) and (iii) are incorrect because in (ii) order of enthalpy of vapourisation is reverse

As size of N is smaller than P, therefore it is difficult to break the bond of NH₃ than PH₃ and AsH₃. So the order of enthalpy of vapourisation should be AsH₃> PH₃> NH₃ .

In (iii) electron gain enthalpy of fluorine is less because of small 2p orbital.

Hence the correct answer is (i) and (iii)