Calculating pH of Na3PO4 and NaH2PO4 Solution with Ka Values for H2CO3

[atavistic]

Homework Statement

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}

Homework Equations

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.

 

[Borek]

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

 

[atavistic]

Actually, there are so many equilibria in this solution I can't figure out what will contribute and the assumption I made(given above) seems to give a wrong answer.

 

[Borek]

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?