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How does carbonic acid cause acid rain when $K_b$ of bicarbonate is greater than $K_a$? Should it not create an alkaline solution?

$K_a = 4.8 \times 10^{-11}\ (mol/L)$. $K_b = 2.3 \times 10^{-8}\ (mol/L)$. These are the values for $\ce{HCO3-}$.

These numbers are from a school book that I read, but it's not in English. It's called "Kjemi 1" by Harald Brandt

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What is the ${K_a}$ of carbonic acid? Note that sources differ in their ${K_a}$ values, and especially for carbonic acid, since there are two kinds - a pseudo-carbonic acid/hydrated carbon dioxide and the real thing (which exists in equilibrium with hydrated carbon dioxide but in a small concentration - about 4% of what what appears to be carbonic acid is true carbonic acid, with the rest simply being $\ce{H2O*CO_2}$.

Either way, I find that the ${K_a}$ of the mixed carbonic acid is about $4.2 \times 10^{-7}$, which is greater than $1.0 \times 10^{-7}$, and this implies that a solution of carbonic acid alone should be acidic no matter what. This suggests to me that your numbers are wrong; would you mind sharing your numbers and their source if possible?

EDIT: I see that you have updated your numbers. Nonetheless, I believe that your ${K_a}$ for carbonic acid is wrong; that number looks suspiciously like the ${K_a}$ instead for hydrogen carbonate ion (or the bicarbonate ion). According to Wikipedia, the ${pKa}$ of carbonic acid, is 6.3 (and this is taking into account any aqueous carbon dioxide). Convert this to a ${K_a}$ value and we get about $5.0 \times 10^{-7}$. This is in-line with the value I obtained from a copy of Daniel C. Harris' Qualitative Chemical Analysis.

EDIT 2: I think you've realized your mistake; as you say, the values are for $\ce{HCO_3^-}$, which is the hydrogen carbonate ion.

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  • $\begingroup$ What do you mean? I asked specifically for HCO3-: "Kb of bicarbonate is greater than Ka?". $\endgroup$ – Ali Mustafa May 8 '14 at 17:31
  • $\begingroup$ Okay, I think we need to revisit your original question about how carbonic acid can make a solution acidic. We need to consider what's in a solution of carbonic acid. In a solution of carbonic acid, we have 1) water and 2) carbonic acid in the main. We do not have bicarbonate ion in a sizeable amount in the solution; therefore, what controls solution pH is carbonic acid. There is no need to consider the various K values for bicarbonate ion because it's simply not in solution at a high concentration relative to carbonic acid - i.e. carbonic acid is a weak acid and does not ionize much. $\endgroup$ – Dissenter May 8 '14 at 17:34
  • $\begingroup$ Okay, but is it H2CO3 or HCO3- that causes acidic rain? My problem is that according to my book, HCO3- + H2O produces an acidic solution, thus giving acidic rain. But at the same time it states that HCO3- will react as a base, because it's Kb >> Ka $\endgroup$ – Ali Mustafa May 8 '14 at 17:50
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    $\begingroup$ True, $HCO_3^-$ will react as both an acid and a base. So bicarb ion is amphoteric. And you are right that bicarb ion is a stronger base than it is acid. I believe that acid rain is caused by carbon dioxide being solvated by water to make "carbonic acid." In this solution of carbon dioxide, there are multiple equilibria, and the reaction of bicarb as an acid is one, but I wouldn't say it's the principle, pH driving reaction. That would be the ionization of "carbonic acid." $\endgroup$ – Dissenter May 8 '14 at 18:59

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