Question

K_c for the reaction; [Ag(CN)₂]^–$\rightleftharpoons$Ag⁺ + 2CN^– , the

equilibrium constant at 25°C is

4.0 × 10^–19, then the silver ion concentration in a solution which was originally 0.1 molar in KCN and 0.03 molar in AgNO₃ is –

• A. 7.5 × 10¹⁸
• B. 7.5 × 10^–18
• C. 7.5 × 10¹⁹
• D. 7.5 × 10^–19

Answer 1

Answer: (B)

7.5 × 10^–18

Answer 2

2KCN + AgNO₃ $\rightleftharpoons$ Ag(CN) + KNO₃ + K⁺ 0.1

0.03 0 0 0

(0.1–0.06) 0 (0.03) (0.03) (0.03)

Ag(CN)= 0.03

Now,

\ Ag(CN)$\rightleftharpoons$ Ag⁺ + 2CN^–

0.03 a 0.04 (left from KCN) (0.03–a) a(0.04 + a)

0.04 + a $\approx$ 0.04

\ K_c = 4 × 10^–19 = $\frac { ( 0.04 ) ^ { 2 } \times \mathrm { a } } { 0.03 }$;

a = 7.5 × 10^–18