Question

When a 20.7 mL sample of a 0.485 M aqueous hydrocyanic acid solution is titrated with a 0.409 M aqueous barium hydroxide solution, what is the pH at the midpoint in the titration? pH =

Answer 1

In an acid base titration, the mid point is that point where exactly half the amount of acids present by the base added.

Now here we have the weak acid Hydrocyanic acid (HCN). Now when we add a strong base like Ba(OH)2, then at mid point, half of he aicds have been converted to its conjugate base.

i.e

HCN (weak acid) + Ba(OH)2 ----------> CN- (conjugate base) + H2O

so we can say at this point, exactly equal amount of HCN and CN- are present. Now since in this both the weak acid and its conjugate base are present, this solution becomes a buffer. And the pH of buffer is calculated by Henderson- Hasselblach equation-

pH = pKa + log [conjugate base] / [weak acid]

Now for HCN, pKa = 9.2

So we can write-

pH = 9.2 + log [CN-] / [HCN]

Now since we found at mid point, [CN-] = [HCN]

So

pH = 9.2 + log 1

= 9.2 + 0

= 9.2

Or simply we can say, at mid point of ttration, pH of the solution = pKa of the acid