Polyprotic acids (H2SO4, H3PO4, H2CO3) dissociate in steps, each with its own Ka. Ka1 >> Ka2 >> Ka3 always, because removing a proton from an increasingly negative ion is progressively harder. For H2CO3: Ka1 = 4.3 x 10^-7, Ka2 = 4.7 x 10^-11. The pH is determined primarily by the first dissociation. For calculating [H+], use Ka1 alone with the original concentration. To find [$CO3^{2-}$], use Ka2 approximately equals [$CO3^{2-}$] (because the second dissociation gives equal increments of H+ and $CO3^{2-}$, and the additional H+ is negligible compared to that from step 1). For the amphoteric ion (HCO3-): pH = $\frac{1}{2}$(pKa1 + pKa2), independent of concentration.