Which acid would be the best choice for preparing a pH = 2.00 buffer? How would you prepare 1.0 L of this buffer?

Question

(a) Which acid would be the best choice for preparing a pH = 2.00 buffer?

HClO₂

HOCl    

HC₂H₃O₂

HCN

HF

How would you use Ka values to calculate this?? Please show the steps.

(b)

Explain how to make 1.0 L of this buffer.

To prepare this buffer, add conjugate base and weak acid in the concentration ratio of ______ parts conjugate base : 1 part weak acid.

How do you use the Ka to find out the ratio of parts?

Answer 1

You need to look up the pKa for each acid, and to make a pH 2 buffer, choose the acid that has a pKa closest to 2. For example, and as a way of showing how to do the calculations, say you find the Ka HOCl = 3.5 x 10^-8 and Ka HF = 6.3 x 10^-4. You can calculate the pKa HOCl to be 7.5 and the pKa HF to be 3.2. In this case HF would be a better choice. Do the same for the other acid that are given as choices and see if any has a pKa closer to 2 than HF.

To make 1.0 L of the chosen buffer with pH = 2, use the Henderson Hasselbalch equation to find the ratio of conjugate base to acid. Thus, pH = pKa + log [conjugate base]/[acid]

2.0 = pKa + log [conjugate base]/[acid] and substitute in the value of the pKa for the chosen acid.

Solve for [conjugate base]/[acid]

Then use the appropriate number of moles of each to attain that molar ratio. Done.