Chemistry
posted by Veronica on .
Determine the concentration of CO32 ions in a 0.18 M H2CO3 solution. Carbonic acid is a diprotic acid whose Ka1 = 4.3 × 107 and Ka2 = 5.6 × 1011.

(CO3^2) = k2. The reasoning is this but it's a little long. I wish we had a board and I could talk.
...........H2CO3 ==> H^+ + HCO3^
I..........0.18......0.......0
C...........x.......x.......x
E.........0.18x.....x.......x
k1 = 4.3E7 = (x)(x)/(0.18x) and solve for x = (H^+) = (HCO3^)if you want an answer but that really isn't necessary. That answer is about 2.8E4M. Then k2 takes over.
...........HCO3^ ==> H^+ + CO3^2
I..........2.8E4....2.8E4...0
C..........x.........x.......x
E.........2.8E4x..2.8E4+x..x
so k2 = (H^+)(HCO3^)/(HCO3^). If you substitute the numbers you have
k2 = (2.8E4+x)(x)/(2.8E4x)
if x is small the k2 = x = (CO3^2)
As above, however, you really don't need to solve for H^+ and HCO3^. Your reasoning tells you (H^+) = (HCO3^) and since k2 = (H^+)(CO3^2)/(HCO3^),they cancel each other in the k2 equation and (CO3^2) = k2 
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