Question

A 50.00 mL sample of a 0.150 M aqueous solution of a weak acid HA is titrated with a 0.300 M aqueous solution of NaOH. At which point in the titration will the pH of the solution equal pKa of the acid?

a) After addition of 50.00 mL of the NaOH solution

b) After addition of 25.00 mL of the NaOH solution

c) After addition of 12.50 mL of the NaOH solution

d) After addition of 6.25 mL of the NaOH solution

Answer 1

The answer is c)after addition of 12.50 mL of the NaOH solution.

When a weak acid is titrated against a strong base like NaOH or KOH the pH become equal to pKa when half the acid's volume of NaOH is added if [NaOH] =[formic acid]

Here the concentration of NaOH is twice tge concentration of HCOOH So volume of NaOH is needed = 50/2×2

= 12.5 mL