PH problem weak acid and weak base

[hsgnfgkid]

Hi

I have been trying several days to solve this problem. But i can't find the procedure to solve this weak acid and weak base problem. I will really appreciate some help.

What is the pH of a soft drink in which the major buffer ingredients are 6.90 g of NaH2PO4 and 5.70 g of Na2HPO4 per 355 mL of solution?

Thanks

 

[chemisttree]

Show us what you have done.

 

[hsgnfgkid]

Hi

The problem relies in the fact i don't know how to solve for weak acid with a weak base. I do know how to buffers with weak acid and salt or weak base and salt. But I have no idea where to start when you are asked a mixture of weak acid and weak base at the same time. I will really appreciate your help. I don't need the final answer just a way to solve this kind of problem.

Thanks

 

[Kalirren]

I think that the conceptual key here is that you can largely ignore the first and third deprotonations of phosphoric acid. If H2PO4(-) and HPO4(2-) exist in significant concentration, then the concentrations of H3PO4 and PO4(3-) will be negligible. That is to say, the only significant equilibrium here is

H2PO4(-) <-> H(+) + HPO4(2-), all phases aqueous.

Then you can just apply the Henderson-Hasselbach equation. Just make sure you use the right Ka.

If you feel paranoid and want to include all of the other equilibria, you can. I'm guessing it'll just be extra work for a 10^-18 or so correction.