Question

$\left[Au\right(CN{)}_2{]}^{\ominus }$ is a very stable complex under certain conditions. The concentration of cyande ion which is required to keep 99 mol$\%$ of the gold in the form of the cyanide complex is: [Given $K_f\left[Au\right(CN{)}_2{]}^{\ominus }=4\times {10}^{28}$]

• A. $2\times {10}^{-28}M$
• B. $3\times {10}^{-14}M$
• C. $5\times {10}^{-28}M$
• D. $5\times {10}^{-14}M$

Answer 1

The correct option is D $5\times {10}^{-14}M$

As given,

$\left[Au\right(CN{)}_2^{\ominus }]\to A{u}^{\oplus }+2C{N}^{\ominus }$

$\Rightarrow K_f=\frac{1}{4\times {10}^{28}}=\frac{\left[A{u}^{\oplus }\right]}{\left[Au\right(CN{)}_2^{\ominus }]}$

$99\%$ of gold in complex form :

$\Rightarrow \frac{\left[A{u}^{\oplus }\right]}{\left[Au\right(CN{)}_2^{\ominus }]}=\frac{1}{99}\Rightarrow [C{N}^{\ominus }{]}^2=\frac{99}{4\times {10}^{28}}$

$\left[C{N}^{\ominus }\right]=5\times {10}^{-14}M$