JEE Main | 2018Chemical EquilibriumHard

Question

An aqueous solution contains 0.10 M H2S and 0.20 M HCl. If the equilibrium constants for the formation of HS- from H2S is 1.0 × 10-7 and that of S2- from HS- ions is 1.2 × 10-13 then the concentration of S2- ions in aqueous solution is :

Options

A.

3 ×  10-20

B.

6 × 10-21

C.

5 × 10-19

D.

5 × 10-8

Solution

[H2S] = 0.10 M
[HCl] = 0.20 M ⇒ [H+] = 0.2 M 
(1) H2S ⇌ HS- + H+   K1 = 1.0 × 10-7
(2) HS ⇌ S2- + H+   K2 = 1.2 × 10-13
So,
H2S ⇌ S2- + 2H+   K1 × K2
 = 1.2 ×10-20
So, [S2-] = 1.2×10-20×[H2S][H+]2
=1.2×10-20×10-14×10-2
= 3 × 10-20 M
All the [H+] will come from strong acid [HCl] only.

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