(a) Account for the following: 

(i) The acidic strength decreases in the order HCl > H₂S > PH₃

(ii) Tendency to form pentahalides decreases down the group in group 15 of the periodic table. 

 (b) Complete the following chemical equations: 

 (i) P₄ + SO₂Cl₂-->

 (ii) XeF₂ + H₂O---> 

 (iii) I₂+HNO₃(conc.)---> 

(i) The acidity of a molecule depends on the polarity of the bond between central atom and the hydrogen atom. Greater the polarity higher will be the acidity.

And the polarity of the bond depends on the electronegativity of the central atom.  In a period, the electronegativity decreases in the order Cl > S > P. As a result, the loss of H⁺ ions decreases. 

 Thus, the acidic strength of the hydrides decreases in the 

 Following order: 

 HCl > H₂S > PH₃

(ii) Nitrogen does not form pentahalide because it does not have d-orbital. P, As, Sb form pentahalide. Bi does not form pentahalide. The tendency to form pentahalide decrease down the group. This because of inert pair effect.
Due to the inert pair effect, ns² electron remains inert in a chemical reaction and element shows -2 oxidation state. Inert pair effect increases down the group. Thus the tendency to form pentahalides decrease down the group 15.    

(b) 

(i) P₄ + 10SO₂Cl₂--->  4PCl₅ + 10SO₂

(ii) 2XeF₂ + 2H₂O---->  2Xe + 4HF + O₂

(iii) I₂ + 10HNO₃--> 2HIO₃+10NO₂+4H₂O