Question

You need to prepare 100.0 mL of a pH=4.00 buffer solution using 0.100 M benzoic acid (pKa = 4.20) and 0.140 M sodium benzoate.

Answer 1

We know that for a buffer solution

pH = pKa + log [salt]/[acid]
4 = 4.20 + log [salt] / [acid]
-0.20 = log [salt]/[acid]

Solve for [salt]/[acid]
This gives ratio of moles.

Take amount of acid be x L and then amount of salt will be 0.1-x
Now molarity*volume=number of moles

Okay I have given a basic idea. Use the details to get to the result. Let me know if you are still facing difficulty.