One mole of P₄ reacts with 8 moles of SOCl₂ to give 4 moles of A, x mole of SO₂ and 2 moles of B . A, B and x respectively are :

Solution

Chemistry ma’am · 3 steps

Do not memorise this one: count S and O on both sides, and only one choice of A and x balances.

1. Set up the count. On the left, P₄ + 8SOCl₂ gives 4 P, 8 S, 8 O and 16 Cl. The products must hold exactly these atoms. With 2 mol of S₂Cl₂ fixed as B, test each option's A and x.

   Option	O on right (need 8)	S on right (need 8)	Balanced?
   (a) 4POCl₃, x = 4	4 + 8 = 12	4 + 4 = 8	no, too much O
   (b) 4PCl₃, x = 4	0 + 8 = 8	4 + 4 = 8	yes
   (c) 4POCl₃, x = 2	4 + 4 = 8	2 + 4 = 6	no, S missing
   (d) 4PCl₃, x = 2	0 + 4 = 4	2 + 4 = 6	no

   P₄ + 8SOCl₂ → 4PCl₃ + 4SO₂ + 2S₂Cl₂

2. Redox check. Chlorine also balances: 12 + 4 = 16. Each P goes from 0 to +3, so 4 P lose 12 electrons. In S₂Cl₂ each S is +1, down from +4 in SOCl₂, so 4 S gain 12 electrons. The electrons match too.
3. So. A is PCl₃, B is S₂Cl₂ and x = 4, which is option (b).

The idea to keep. When a question gives some mole numbers and hides a product, use atom balance on each element. The element that appears in only a few species, here S and O, catches the wrong options fastest.

Answer from the NTA final answer key. Solution by PrepWiser.