Question

For hydrocyanic acid, HCN, Ka = 4.9 × 10-10. Calculate the pH of 0.20 M NaCN. What is the concentration of HCN in the solution? for the ph i got 10 is that right?

Answer 1

Answer : The concentration of HCN in the solution is
`2.0* 10^(-3)M`

Explanation :

First we have to calculate the value of
`pK_a`.

The expression used for the calculation of
`pK_a` is,

`pK_a=-\log (K_a)`

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

`pK_a=-\log (4.9* 10^(-10))`

`pK_a=10-\log (4.9)`

`pK_a=9.3`

Now we have to calculate the pH of the solution.

The hydrolysis reaction will be,

`NaCN+H_2O\rightarrow HCN+NaOH`

Formula used :

`pH=7+(1)/(2)[pKa+\log C]`

`pH=7+(1)/(2)[9.3+\log (0.20)]`

`pH=11.3`

Now we have to calculate the concentration of HCN in the solution.

Using Henderson Hesselbach equation :

`pH=pK_a+\log ([Salt])/([Acid])`

`pH=pK_a+\log ([NaCN])/([HCN])`

Now put all the given values in this expression, we get:

`11.3=9.3+\log ((0.20)/([HCN]))`

`[HCN]=2.0* 10^(-3)M`

Therefore, the concentration of HCN in the solution is
`2.0* 10^(-3)M`