# Dicarboxylic acid titration

1. Aug 23, 2014

### 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

2. Aug 23, 2014

### Staff: Mentor

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.

3. Aug 23, 2014

### nmr

so is my pH value correct ?

4. Aug 23, 2014

### Staff: Mentor

Nope.

5. Aug 23, 2014

### nmr

how would i go about solving for the pH then?

6. Aug 23, 2014

### Staff: Mentor

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.

7. Aug 23, 2014

### nmr

ah ok sorry

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