# Another pH

1. Feb 13, 2009

### atavistic

1. The problem statement, all variables and given/known data

$$50ml$$ of $$0.10M Na_3PO_4 + 50ml$$ of $$0.10M NaH_2PO_4$$

For $$H_2CO_3$$:
$$Ka_1 = 7.5 * 10^{ - 3}$$
$$Ka_2 = 6.2 * 10^{ - 8}$$
$$Ka_3 = 1.0 * 10^{ - 12}$$

2. Relevant equations

3. The attempt at a solution

Since $$K_h$$ of $$Na_3PO_4 > > Ka_2$$ , I considered only hydrolysis of $$Na_3PO_4$$ but it gives wrong answer.

2. Feb 14, 2009

### Staff: Mentor

You have a mixture of a weak acid and its conjugate base - does it ring a bell?

3. Feb 14, 2009

### atavistic

Actually, there are so many equilibria in this solution I cant figure out what will contribute and the assumption I made(given above) seems to give a wrong answer.

4. Feb 14, 2009

### Staff: Mentor

Actually my previous comment was not too good

Think this way: you have mixed equimolar amounts of Na3PO4 and NaH2PO4. Does it differ from just dissolving Na2HPO4?