Question

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

Answer 1

Ok let me explain the procedure to you:
The buffer solution is like this:
pH = pKa + log [salt]/[acid] 4 = 4.20 + log [salt] / [acid] -0.20 = log [salt]/[acid]
If you 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
I know that with this you will be able to get to the result. Hope this helps