[bren12310]

A sample of 0.1587 g of an unknown monoprotic acid was dissolved in 25.0 mL of water and titrated with 0.1150 MNaOH. The acid required 15.5 mL
of base to reach the equivalence point.
   
Part A (already got this part)

What is the molecular weight of the acid?
Express your answer using three significant figures.

89.0 g/mol (this is the correct answer)


Part B

After 7.25 mL
of base had been added in the titration, the pH was found to be 2.75. What is the Ka for the unknown acid? (Do not ignore the change, x, in the equation for Ka.)

[hypervalent_iodine]

As per the rules of this forum, you will need to show some attempt at this before anyone can help you. IOW, what have you done and where exactly are you stuck?

ETA: Do you know the correct answer for b.? Unless I'm missing something or you haven't learnt about certain equations yet, "Do not ignore the change, x, in the equation for Ka," seems like it could be leading you down the garden path a bit.