Question

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.

Answer 1

a) The pKa of the reaction H₂PO₄^- ? H⁺ + HPO₄^2- is 7.21 (source wikipedia http://en.wikipedia.org/wiki/Phosphoric_acid)

Thus using the Henderson-Hasselbalch equation we arrive at [HPO₄^2-]/[ H₂PO₄^- ] = 0.1

Therefore, 0.1 M phosphate buffer at pH 6.2 will have 9.1% of  H₂PO₄^- and 91% of HPO₄^2- 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.