Analytical Chemistry
posted by Sam on .
Phosphoric acid is a triprotic acid with the following pKa values:
pka1: 2.148 pka2: 7.198 pka3: 12.375
You wish to prepare 1.000 L of a 0.0100 M phosphate buffer at pH 7.45. To do this, you choose to use mix the two salt forms involved in the second ionization, NaH2PO4 and Na2HPO4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture?
?? grams NaH2PO4
?? grams Na2HPO4
Hint: Use the HendersonHasselbalch equation to get the molar ratio of Na2HPO4 to NaH2PO4 required, then the fraction of each form from the ratio. The total moles needed will be 1.000 L × 0.0100 M = 0.0100 moles. Use the formula mass to calculate the mass needed. (FM NaH2PO4 = 119.98; FM Na2HPO4 = 141.96).
I am completely lost on how to solve this problem.

pH=pka+log({A]/[HA])
pH=7.45
pka=12.375
Solve for ratio:
10^(7.4512.375)={A]/[HA]=1.19x10^5
Since you have a total of 0.01 moles,
1.19x10^5*(0.01moles)= moles of Na2HPO4
0.01 molesmoles of Na2HPO4= moles of NaH2PO4
Use the formula weights to solve for the number for each that Dr. Bob222 gave you.
**** Not sure about that pKa value that I chose. 
The correct pKa value to choose is pK2.
Use the HH equation to solve for the ratio base/acid.
One equation you need is base/acid.
The other equation you need is
base + acid = 0.01
That two equations in two unknowns; solve for acid concn and base concn and convert to grams. Post your work if you gets stuck. 
Setup is correct, but substitute 7.198. That should change 1.19x10^5 to 1.78 and do what Dr.Bob222 told you to do.

if you use the pk2 wouldnt you get a negative value for nah2po4?

After you get the ratio. From the
0.01 molesmoles of Na2HPO4= moles of NaH2PO4
=> (0.01  Na2HPO4)/(Na2HPO4) = (ratio of pka2 from hasselbalch)
unit will be mol, convert to g