Question

Five hundred litres (500 L) of a solution 0.0100 M in $\left[Au\right(CN{)}_2{]}^{\ominus }$ and 0.0030 M in $\left[Ag\right(CN{)}_2{]}^{\ominus }$ was evapourated to one-third of the original volume and was treted with zinc (40 g), assuming that deviation from standard conditions is unimportant in this case and all the concerned redox reaction go essentially to completion. Hence, the concentration of $\left[Au\right(CN{)}_2{]}^{\ominus }$ and $\left[Ag\right(CN{)}_2{]}^{\ominus }$ after reaction are:

• A. $\left[Au\right(CN{)}_2{]}^{\ominus }=0.030M;\left[Ag\right(CN{)}_2{]}^{\ominus }=0.003M$
• B. $\left[Au\right(CNB{)}_2{]}^{\ominus }=0.0100M;\left[Ag\right(CN{)}_2{]}^{\ominus }=0.0090M$
• C. $\left[Au\right(CN{)}_2{]}^{\ominus }=0.030M;\left[Ag\right(CN{)}_2{]}^{\ominus }=0.0020M$
• D. $\left[Au\right(CN{)}_2{]}^{\ominus }=\left[Ag\right(CN{)}_2{]}^{\ominus }=0.030M$

Answer 1

Answer: (C)

$\left[Au\right(CN{)}_2{]}^{\ominus }=0.030M;\left[Ag\right(CN{)}_2{]}^{\ominus }=0.0020M$

Zn will react with both $\left[Au\right(CN{)}_2{]}^{\ominus }$ and
$\left[Ag\right(CN{)}_2{]}^{\ominus }$ as it is more active metal.

$Zn+2Ag(CN{)}_2^{\ominus }\to [Zn(CN{)}_4{]}^{2-}+2Ag$

$\Rightarrow ConcofAg(CN{)}_2^{\ominus }=\frac{0.3}{500/3}=0.022M$

and concentration of $Au(CN{)}_2^{\ominus }=0.01\times 3=0.03M$