One strategy for dealing with the acidification of lakes is periodically to add powdered limestone (CaCO3) to them. Calculate the pH of a lake that has more than enough powdered limestone in it to saturate the water with its ions Ca2+ and CO32-. The [CO2] in the lake is 1.2*10-5 mol/L. Suggestions: Begin with the carbonate systems equations and then add a charge balance equation: [H+]+2[Ca 2+] = [HCO3 -]+2[CO3 2-]+[OH-] appr. = [HCO3 -]+[OH-] The preceding holds for pH values that are less than about 10, when you work out the solution check to see whether this was a valid assumption.