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Amount of Calcium Carbonate Dissolved

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The average pH of precipitation in rural areas is 6.80. Assuming that the pH
Is controlled by the carbonate system, i.e., no anthropogenic acid gases are present in the atmosphere; calculate the amount of calcium carbonate (mg) dissolved per liter of rainwater. pKa1=6.33, KCO2=3.388e-02

I've seen a couple different solutions to these problems and all of them use other constants and numbers that I have no idea where they got them from. Please explain.

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For this question, you must understand that there are many related reactions going on.

CO2 dissolving into water...

CO2 + H2O --> H2CO3

producing the carbonic acid...

Carbonic acid dissociates into.....

H2CO3 --> HCO3- + H+

bicarbonate....which also dissociates (since carbonic acid is diprotic we see two dissociations)

HCO3- --> CO3^2- + H+

Then we have the solubility of calcium carbonate involved....

CaCO3 --> Ca^2+ + CO3^2-

The only pH dependent component of the solubility of CaCO3 is the carbonate (CO3^2-) ...

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