Preparing 700 mL PBS buffer with Henderson-Hasselbalch equation

[WSR]

Question

You are asked to prepare 700 mL of PBS (phosphate buffered saline). The PBS needs to be pH 7.40 with 25mM phosphate and 140 mM NaCl. You are given a bottle of NaCl (58.45 g/mol), 1 M Na2HPO4 stock solution and a 1.00 M NaH2PO4 stock solution. ( H3PO4 pk1=2.12, pk2=7.21, pk3=12.66) Describe how to prepare this buffer.


Equations

I'll use the Henderson-Haeelbalch Equation so pH= pka +log(base/acid)


The thing that really screws me up in this problem is how to incorporate the M concentrations. The pH has to be 7.40 so 7.40 = pka + log (base/acid). I don't know where to go with the concentrations.


Also, a separate question that I don't know if I did correctly.
At what pH will 62% of x be in the protonated form if pka = 8.10. So I did, pH= pka +log(base/acid) and got pH= 8.10 + log (.38/.62) with an answer of 7.89. Does that makes sense?

I'd appreciate any guidance.

Thanks

 

[Borek]

You know sum of concentrations of hydrogen and dihydrogen phosphates must be 25 mM. That, plus Henderson-Hasselbalch equation give you two equations in two unknowns - solve for both concentrations. Then calculate how much of the original solutions you need to dilute to produce 700 mL of the PBS.

Approach to the other question looks OK, I haven't checked numbers.