A 0.25-mol sample of a weak acid with an unknown Pka was combined with 10.0-mL of 3.00 M KOH, and the resulting solution was diluted to 1.500 L. The measured pH of the solution was 3.85. What is the pKa of the weak acid?

Answer : The value of of the weak acid is, 4.72

Explanation :

First we have to calculate the moles of KOH.

Now we have to calculate the value of of the weak acid.

The equilibrium chemical reaction is:

Initial moles     0.25     0.03        0

At eqm.    (0.25-0.03)   0.03      0.03

                     = 0.22

Using Henderson Hesselbach equation :

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

Therefore, the value of of the weak acid is, 4.72

the answer is c i believe

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