Buffer Preparation Problem

[Erinyes83]

Got a tough problem I have no clue how to do.

I have to prepare 500mL of a buffer solution with a pH of 12.50 and must be at least .10 M.

The only reagents I have are:
solid sodium dihydrogen phosphate dihydrate NaH2PO4 x (H2O)2
Distilled water
1.00 M NaOH(aq)
1.00 M HCL(aq)

Not all reagents need to be used. If I could get some help on how to do this problem, anything would be aprreciated.

[Monique]

did you try the henderson-hasselbach equation?

[iluvsr20s]

Hederson-Hasselbach wouldn't work because you dont have any of the equillibrium constants such as Ka and Kb
I think its impossible because strong acids and strong bases dont create buffer solutions

[Monique]

Equilibrium constants can be looked up in chemistry books, and the NaH2PO4 is the buffering agent. The strong acid and strong base are for equilibrating it to the right buffering pH.

[iluvsr20s]

I see it now, my apologies, Henderson-Hasselbach would be in order. I believe the NaH2PO4 would ionize in water to form Phosphoric Acid and Sodium Phosphate. Then of course using the equation aformentioned to determine the pH. Then I'm afraid that i am baffled, because that would be an acidic buffer solution, and following a rule that was taught to me, an acid with 2 more oxygens then hydrogens is a strong acid, but there are exceptions and this may be one of them.