Question

$K_{c}$ for the the reaction, $\left[ Ag ( CN )_{2}\right]^{-} \rightleftharpoons Ag ^{+}+2 CN ^{-}$, the equillibrium constant at $25^{\circ} C$ is $4.0 \times 10^{-19}$, then the silver ion concentration in a solution which was originally $0.1$ molar in $KCN$ and $0.03$ molar in $AgNO _{3}$ is :

• A. $7.5 \times 10^{18}$
• B. $7.5 \times 10^{-19}$
• C. $7.5 \times 10^{19}$
• D. $7.5 \times 10^{-18}$

Answer 1

Answer: (D)

$7.5 \times 10^{-18}$

Answer 2

$0.04 \approx 0.04$
$\therefore \,K_c = 4 \times 10^{-19} = \frac{(0.04)^2 \times a}{0.03}$
$a = 7.5 \times 10^{-18}$