chemistry

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A) Calculate the hydrogen-ion concentration of a 5.45*10^-4 M H2CO3, solution, noting that Ka is bigger than Kb.
B)What is the concentration of CO3^2-?

after finding the concentration to A), do i use that concentration (M) as the initial M for CO3^2-?

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To be honest I don't understand the caution in the problem. I also think the concn of the H2CO3 is so low that it may cause problems but it didn't have that much effect on the final calculation.
H2CO3 ==> H^+ + HCO3^-
Set up an ICE chart, use k1 ONLY, (just think of it as a monoprotic acid and work the H^+ that way). It PROBABLY is a good idea to include the quadratic eqution and solve that.
[(H^+)(HCO3^-)/(H2CO3-(H^+)] = k1 which becomes
[(x)(x)/(H2CO3-x)] = k1.

For part b, the trick here is to think of this diprotic acid as a monoprotic acid. Then (H^+) = (HCO3^-)
Now look at k2 expression.
k2 = (H^+)(CO3^2-)/(HCO3^-)
BUT, if (H^+) = (HCO3^-) then (H^+) in the numerator cancels with (HCO3^-) in the denominator and k2 = (CO3^2-)