How Do You Calculate the pH at Equivalence Points in Malonic Acid Titration?

[nmr]

Malonic acid can ionise in two stages as it is a dibasic acid.
The values of pKa1 and pKa2 of malonic acid are 2.85 and 5.70 respectively. Calculate the pH of the first and second equivalence points of the titration of 10cm^3 of malonic acid of concentration 0.1 M with sodium hydroxide of concentration 0.1 M.

my attempt at a solution:
maximum buffering capacity before first endpoint: pH=pKa1 = 2.85

[OH-] = [(Kw/Ka1)*(0.05)]^(0.5)
therefore pOH = 6.23
pH = 14-6.23 = 7.77

im stuck here as the answer given for the pH of the first endpoint is 3.50 and I have no idea how to work it out

 

[Borek]

First endpoint pH given as an answer is wrong, so don't worry if you can't reproduce it.

At first endpoint you have a solution of an amphiprotic salt.

 

[nmr]

so is my pH value correct ?

 

[Borek]

Nope.

 

[nmr]

how would i go about solving for the pH then?

 

[Borek]

I have pointed you in the right direction in my first answer.

If you plan to ignore hints please tell, so that I don't waste my time answering.

 

[nmr]

ah ok sorry