Question

You titrated a weak acid with a strong base. The equivalence point was reached after 20 mL of base was added. After adding how many mL of base is the pH of the solution equal to the pKa?

Please answer correctly. I will give you thumbs up (+1)

Answer 1

The equivalence point represents we have added sufficient amount of OH- to react with the H+ present of the acid.

Number of moles of acid = Number of moles of Base

The reaction of weak baes will be

HA + OH- ============= A- + H2O

Using hessley henderbach equation

pH = pKa + log[A-/HA]

Since we want pH = pKa, we need to have [A-] = [HA], which implies we want the reaction to reach half the equivalence point

Hence volume of base added must be 1/2 * 20 mL = 10 mL

So therefore after adding 10mL of the base, the pH of the solution will be equal to pka