Question

A 100-mL solution of weak acid was titrated with 0.09145 M NaOH solution. The titration showed that Ve/4 = 8.55 mL, and the measured pH at that volume was 5.00. Determine the pH at the equivalence point, and choose an appropriate endpoint indicator. Hint: recall that the H-H equation does not apply at the equivalence point.

Answer 1

Given that Ve / 4 = 8.55 mL, where Ve is the equivalence volume

The pH at the above volume = 5

i.e. Ve = 4*8.55 = 34.2 mL of NaOH

The volume is inversely proportional to concentration

The four times increase in volume corresponds to 4 times decrease in the concentration.

i.e. pH = -Log([HA]/4)

= -Log[HA] + Log4

= pH + 0.6

= 5 + 0.6

Therefore, the pH at the equivalence volume = 5.6

Here, the suitable end point indicator is phenol red or phenalphthalein (HPh), because the given mixture is a combination of weak acid/strong base.