[Kritkatten]

Hello!
I'm working on solubility and how pH affects a solution. Surfing the world wide web for information, I came upon this example, which was using something called K_sum. Which is nothing I have ever known about, and the example looked like this:

(1)    CaCO₃(s)   Ca²⁺(aq) + CO₃^2–(aq)   K_sp(CaCO₃)=3.36x10^–9
(2)    CO₃^2–(aq) + H₃O⁺(aq)   HCO₃^–(aq) + H₂O(l)   1/K_a2(H₂CO₃)=1/4.7x10^–11
Sum  CaCO₃(s) + H₃O⁺(aq)   Ca²⁺(aq) + HCO₃^–(aq) + H₂O(l)   K_sum

K_sum = K_sp(CaCO₂)/K_a(HCO₃^–) = 3.36×10^–9 / 4.7×10^–11 = 71.5.


I understand the summation of the reactions, but I do not understand K_sum, what it is and why it is division and not addition...
Can anyone please explain this to me?