The Best Answer for Calculate the concentration of all species in a 0.180 M solution of H2CO3?
Tricky. That's because there are NO molecules of H2CO3 in a solution of carbonic acid. Molecules of H2CO3 do not exist in liquid water. Carbonic acid is actually a solution of CO2 in equilibrium with small amounts of H+ ions and HCO3^- ions. The equilibrium lies far to the left, which means that most of the CO2 stays CO2.
CO2(aq) + H2O(l) <==> H+ + HCO3^-
HCO3^- is a weak acid, so there will be even less CO3^2- in solution.
HCO3^- <==> H+ + CO3^2-
CO2(aq) + H2O(l) <==> H+ + HCO3^- ........... Ka = 4.3x10^-7 (*)
Ka = [H+][HCO3^-] / [CO2]
4.3x10^-7 = x² / 0.180
x = 2.8x10^-4 .... x = [H+] = [HCO3^-]
[H+] = 2.8x10^-4 ...... pH = 3.57
[OH-] = 1.0x10^-14 / 2.8x10^-4 = 3.6x10^-11
HCO3^- <==> H+ + CO3^2- ...... Ka = 4.8x10^-11
Ka = [H+][CO3^2-] / [HCO3^-]
4.8x10^-11 = [CO3^2-]