### Final Exam Fall 2012 #8

Posted:

**Sun Dec 06, 2015 11:49 am**Question: An approximation to finding the pH of a solution containing a polyprotic acid is to treat each step independently and add the [H+] generated at each step. Using this apprximation find the pH of a 0.0037 M solution of H2CO3 using the given equations:

H2CO3 -> H+ + HCO3- Ka1 = 4.3 x 10^-7

HCO3- -> H+ + (CO3)2- Ka2= 5.6 x 10^-11

Why do we have to use two separate equations to solve for pH. Why not use the Ka total?

H2CO3 -> H+ + HCO3- Ka1 = 4.3 x 10^-7

HCO3- -> H+ + (CO3)2- Ka2= 5.6 x 10^-11

Why do we have to use two separate equations to solve for pH. Why not use the Ka total?