I need a phosphate buffer with a pH of 75 and I have NaH2PO4

I need a phosphate buffer with a pH of 7.5 and I have NaH2PO4 (acidic salt) and Na2HPO4 (salt with conjugate base) available. What concentration of each do I need? Hint: Use the Henderson-Hasselbalch equation below. Set acid concentration to 1 The pKa of H2PO4- is 7.21.

Solution

pH = pka +log (acid/salt )

7.5 =7.21 +log (acid/salt )

(acid/salt ) =e^0.29=1.336

acid conc = 1.336 salt conc