An unknown acid is titrated with a strong base. At the half-equivalence point (i. e., at the volume halfway to the equivalence point), the pH is 4.2. What is the pKa of the unknown acid?

Answer : The  pKa of the unknown acid is, 4.2

Explanation : Given,

pH = 4.2

As we know that, at half-equivalence point the moles of salt formed is equal to the moles of acid remaining.

Using Henderson Hesselbach equation :

Now put all the given values in this expression, we get:

Therefore, the pKa of the unknown acid is, 4.2

it's all four options.