MCQ Solution: Chemical Equilibrium
Equilibrium constant for the reaction:
Ag+ + 2CN- ⇌ [Ag(CN)2]-
at 25oC is 2.5 × 1018. Calculate silver ion concentration (in terms of 10-17 M) in a solution which was originally 0.10 M in KCN and 0.03 M in AgNO3.
1) 0.75
2) 1.85
3) 0.61
4) 1.24
SOLUTION
Since equilibrium constant for the formation (stability constant) of the complex is of the order of 1018, almost all Ag+ ions will enter into complex ion formation.
Hence, [Ag(CN)2]- = 0.03 M
[CN-] = 0.1 - 2 × 0.03 = 0.04 M
K = $$\cfrac{[Ag(CN)_{2}^{-}]}{[A{{g}^{+}}]{{[C{{N}^{-}}]}^{2}}}$$ = 2.5 × 1018 (given)
[Ag+] = $$\cfrac{[Ag(CN)_{2}^{-}]}{{{[C{{N}^{-}}]}^{2}}\times 2.5\times {{10}^{18}}}$$
= $$\cfrac{0.03}{{{0.04}^{2}}\times 2.5\times {{10}^{18}}}$$ = 0.75 × 10-17 M
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