I did this question but I got 4.18 as the pH. I'm confused to how you solve this and get the right answer.

This a question in which NaOH is added to weak acid lets say HA.
First write the balanced equation and since you are mixing volumes so there is a change in molarity so work in mmol to check after the reaction what all is left.
      R  NaOH + HA ---> Na⁺ A^-  + H2O(l)
                    I   15    25          0                   
                    F    0    10          15                         -
Finally in the solution you have weak acid and its conjugate base. So its a Buffer and use Henderson Hasselbach  equation to calculate the pH.
pH is 4.18

I think the correct answer was marked as 2.62 on the quiz. I put 4.18 and it said it was incorrect.

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