[trinhn812]

malonic acid is a diprotic acid. in the titration of malonic acid with NaOH, stoichiometric points occur at pH 3.9 and 8.8
A 25.00 ml sample of malonic acid of unknown concentration is titrated with .0984 M NaOH requiring 31.50 mL of the NaOH soultion to reach the phenophthalein end point. calculate the concentration of malonic acid in the unknown solution.

At pH =8.8
H+ = 1.65e-9 = let's label this as x
Okay so I think the equation is
  A- + H20 = HA + OH-

A- is  y/(31.50mL+31.50mL) = z

Ka=(x)(x)/ z  

So the problem I'm having is, what is Ka? And why are there two stoichiometric points? Do I just pick one for x?

[Borek]

You start with H₂A - try to imagine how it is neutralized. Will HA^- neutralization start before H₂A is neutralized?