Utilizing pH to get concentrations

a. What is the concentration of a 75 ml solution of benzoic acid with a pH of 3.9? (pKa = 4.2)
b. If the current ocean pH is 8.1, what is the most likely ratio of H2CO3/HCO:-/CO32- present? pKai = 6.4; pKaz = 10.3
c. One proposed solution to ocean acidification is to, essentially, add a base back to the ocean. If we were to try to change the pH of the Atlantic Ocean (roughly 3 x 1020 L) to 8.2, how many grams of NaOH would we have to add if there is a 2.3 mM carbonic acid buffer current at a pH of 8.1? (Assume any volume change is negligible).