Question

Describe how to prepare 50 mL of solution buffered at pH 5.00. The buffer must be designed so the pH changes no more than 0.10 units with the addition of 0.001 moles of acid or base. Choose from the following to design the buffer. Be sure to choose the buffer pair best suited to the assigned conditions.

1.acetic acid/sodium acetate

2.formic acid/sodium formate

3. benzoic acid/sodium benzoate

please show your work-thank you!

Answer 1

(1) You need to pick an acid/base pair with a pKa that is 5 +/- 1 since buffers work best if their pKa is within 1 pH unit of the target pH.
(2) You will then need to calculate the mmol base/mmol acid ratio needed for the buffer. This can be accomplished by the Henderson-Hasselbalch equation where pH = pKa + log(mmol base/mmol acid).
(3) The next step is determining the actual quantities of base and acid needed. Let's say you are testing the buffer by adding 1 mmol of acid. The pH will then go down by 0.10 to a final pH of 4.90 since the base/acid ratio changes. So, the Henderson-Hasselbalch equation for this change is shown as:
4.90 = pKa + log[(mmol base - 1 mmol)/(mmol acid + 1 mmol)]. Since we already know that (mmol base)/(mmol acid) = the ratio you calculated in (2), we now have two equations in two unknowns. Use some algebra to solve the two equations for the mmol acid and mmol base.
(4) Now that you know the molar quantities of acid and base needed to make the buffer, you can convert these to grams (if the compound is solid) or mL (if the compound is a solution of known molarity).