Answer : The concentration of HCN in the solution is 
Explanation :
First we have to calculate the value of
.
The expression used for the calculation of
is,

Now put the value of
in this expression, we get:



Now we have to calculate the pH of the solution.
The hydrolysis reaction will be,

Formula used :
![pH=7+\frac{1}{2}[pKa+\log C]](https://tex.z-dn.net/?f=pH%3D7%2B%5Cfrac%7B1%7D%7B2%7D%5BpKa%2B%5Clog%20C%5D)
![pH=7+\frac{1}{2}[9.3+\log (0.20)]](https://tex.z-dn.net/?f=pH%3D7%2B%5Cfrac%7B1%7D%7B2%7D%5B9.3%2B%5Clog%20%280.20%29%5D)

Now we have to calculate the concentration of HCN in the solution.
Using Henderson Hesselbach equation :
![pH=pK_a+\log \frac{[Salt]}{[Acid]}](https://tex.z-dn.net/?f=pH%3DpK_a%2B%5Clog%20%5Cfrac%7B%5BSalt%5D%7D%7B%5BAcid%5D%7D)
![pH=pK_a+\log \frac{[NaCN]}{[HCN]}](https://tex.z-dn.net/?f=pH%3DpK_a%2B%5Clog%20%5Cfrac%7B%5BNaCN%5D%7D%7B%5BHCN%5D%7D)
Now put all the given values in this expression, we get:
![11.3=9.3+\log (\frac{0.20}{[HCN]})](https://tex.z-dn.net/?f=11.3%3D9.3%2B%5Clog%20%28%5Cfrac%7B0.20%7D%7B%5BHCN%5D%7D%29)
![[HCN]=2.0\times 10^{-3}M](https://tex.z-dn.net/?f=%5BHCN%5D%3D2.0%5Ctimes%2010%5E%7B-3%7DM)
Therefore, the concentration of HCN in the solution is 