This is a question that I came across that I don't fully understand the solution for:
"You have been supplied with 2M solution of carbonic acid (H2CO3) and a 6M solution of NaOH. What
volumes of these materials are required to prepare 1 L of a 50 mM buffer solution at pH 7 (pKa for
carbonic acid is 6.35)?"
My solution:
Using Henderson-Hasselbalch equation, the ratio of HCO3- to H2CO3 is 4.47:1. With a system of equations I solved for the concentration of each, getting [HCO3-] = 0.041 M and [H2CO3] = 0.009 M.
Then I stated that [HCO3-] = [NaOH] = 0.041 M.
Using the dilution formula I found that 6.8 mL of NaOH are required.
Part 2:
For the volume of H2CO3, I found using the same method of dilution that 4.5 mL of H2CO3 are required.
However, the solution states that 25 mL of H2CO3 are required, rather than 4.5 mL. This seems to suggest that [H2CO3] = 0.05 M, but this is clearly not the case...?