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Amount of Calcium Carbonate Dissolved

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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