Answer : The pH at equivalence is, 9.21
Explanation : Given,
Concentration of
= 0.5870 M
Volume of
= 90.0 mL = 0.09 L (1 L = 1000 mL)
First we have to calculate the moles of



As we known that at equivalent point, the moles of
and NaOH are equal.
So, Moles of NaOH = Moles of
= 0.0528 mol
Now we have to calculate the volume of NaOH.



Total volume of solution = 0.09 L + 0.0253 L = 0.1153 L
Now we have to calculate the concentration of KCN.
The balanced equilibrium reaction will be:

Moles of
= 0.0528 mol

At equivalent point,
![pH=(1)/(2)[pK_w+pK_a+\log C]](https://img.qammunity.org/2021/formulas/chemistry/college/cvxc4ri024tdu4yedmekcr3wix0gtv39cv.png)
Given:

Now put all the given values in the above expression, we get:
![pH=(1)/(2)[14+4.76+\log (0.458)]](https://img.qammunity.org/2021/formulas/chemistry/college/xa85gqappj52gp3al3os02dmu7i80qi955.png)

Therefore, the pH at equivalence is, 9.21