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
