Environmental science- Alkalinity homework

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Question description

Im stuck on four questions in my Environmental Chemistry assignment that i just cant figure out, which is super frustrating because am pretty sure they are relatively basic! Need to know how to do the working, not so much what the final answers are.. Below each question is my working so far. Here goes:

4) The equilibrium between dissolved carbon dioxide and calcium carbonate minerals determines the alkalinity, pH and dissolved calcium concentrations of water. Assume a concentration of carbon dioxide in water in equilibrium with air of 1.276 × 10-5 M. Then, use the solubility product of calcium carbonate (calcite) and the acid dissociation constants for carbon dioxide and bicarbonate to calculate the pH of water in that is in equilibrium with solid calcium carbonate. Show each step in your calculation.

Me: CO2 concentration: 1.276 × 10-5 M Acid dissociation constant for CO2: 4.45 × 10-7 Acid dissociation constant for HCO3-: 4.69 × 10-11 Solubility product of calcite (CaCO3): 4.47 × 10-9

5) Water with an alkalinity of 2.00 × 10-3 eq/L has a pH of 7.0. Calculate [CO2], [HCO3-], [CO32-], and [OH-].

Me: The contributions made by different chemical species to alkalinity depend upon pH. At pH 7.00, [OH-] is too low to contribute to the waters alkalinity. Due to CO23- + H+ -> HCO3-, at pH 7.00 [HCO3-] >> [CO23-]. So the alkalinity is due to HCO3-, [HCO3-] at pH7 is 1.00 x 10-3 (the natural alkalinity of atmospheric water). Ka1 expression of the alkaline action in water of common ions: Ka1 = [H+][HCO3-]/[CO2] Ka1 = [H+]2/1.146 × 10-5 (H+ squared as assuming that HCO2 and H+ are the same) Ka1 = 4.45 × 10-7 [H+] = [HCO3-] = (1.146 × 10-5 × 4.45 × 10-7)1/2 = 2.25 × 10-6 * THIS CALCULATES TO 2.25 × 10-12? pH = 5.65 This calculation explains why pure water that has equilibrated with the unpolluted atmosphere is slightly acidic with a pH

6) For a solution having 1.00 × 10-3 eq/L total alkalinity (contributions from HCO3-, CO32-, and OH-) at [H+] = 4.69 × 10-11, what is the percentage contribution to alkalinity from CO32-?

Me: Lost ???

8) Assume the atmosphere contains 390 ppm carbon dioxide. What is the pH of rainwater due to the presence of this carbon dioxide? What would be the pH of rainwater if the atmosphere contained twice as much carbon dioxide?

Me: Lost ??? Gather I need the pKa of CO2 which is 6.35 (-log 4.45x10-7, the Ka or acid dissociation constant for CO2)

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