Question

# A) Need to prepare 100 ml of a 0.10 M phosphate buffer with a pH of...

A) Need to prepare 100 ml of a 0.10 M phosphate buffer with a pH of 6.2. Calculate the amounts of 1.0 M NaH2PO4 and 1.0 M K2HPO4 solutions which will give the required pH of 6.2.Use the Henderson-Hasselbalch equation ( pH = pKa + log (base/acid)

B) Need to prepare 100 ml of a solution containing 0.025 M NaHCO3 and 0.12 M NaHCO3 and 0.12 M NaCl From stock solutions of 0.05 M NaHCO3 and 0.24 M NaCl from stock solutions of 0.05 M NaHCO3 and 0.24 M NaCl. Calculate how much of each solution you will need.

a) The pKa of the reaction H2PO4- ? H+ + HPO42- is 7.21 (source wikipedia http://en.wikipedia.org/wiki/Phosphoric_acid)

Thus using the Henderson-Hasselbalch equation we arrive at [HPO42-]/[ H2PO4- ] = 0.1

Therefore, 0.1 M phosphate buffer at pH 6.2 will have 9.1% of  H2PO4- and 91% of HPO42- OR in 100ml of buffer 0.0091 mol and 0.091 mol respectively.

Thus mixing 91 ml of 1.0 M K2HPO4 and 9.1 ml of 1.0 M NaH2PO4 solution results in 100 ml of 0.1 M phosphate buffer

b) I think the question in correct form is 100 ml of a solution containing 0.025M NaHCO3 and 0.12 M NaCl from stock solutions of 0.05 M NaHCO3 and 0.24 M NaCl.

Since the required solution contains the concentration half of the stock solution, mixing of 50 ml each from the stock solution will result 100 ml of solution containing 0.025M NaHCO3 and ).12M NaCl.