Carbon dioxide in the air dissolves, and the dissolved carbon dioxide undergoes

Question

Carbon dioxide in the air dissolves, and the dissolved carbon dioxide undergoes acid-base reactions:

a. If the partial pressure in the atmosphere is 10-3.5 atm and KH= 10-1.5 moles/L-atm, calculate the concentration of CO2(aq).

b. After all the reactions reach equilibrium, will the pH be greater than, less than, equal to 7?

c. In a separate system, CaCO3(s) dissolves in water: CaCO3(s) <--> Ca2+ + CO32-. Carbonte ion undergoes further reaction in the water. After all the reactions reach equilibrium, will the pH be greater than, less than, equal to 7?

Explanation / Answer

a). [CO2] = 10-3.5 * 10-1.5 = 10-5 mol/L

[CO2] (ppm) = 1x10-5 mol/L * (44 g/mol) * 1000 mg/g = 0.44 mg/L

b) Assuming concentration of 1x10-5 M and that CO2 dissolves completely in water, the concentration of this, would be the same as H2CO3 so: (K1 = 4.5x10-7; K2 = 4.8x10-11)

H2CO3 -------> H+ + HCO3-

[H+] = (K1K2F + K1Kw / K1 + F)1/2

[H+] = (4.5x10-7 * 4.8x10-11 * 1x10-5 + 1x10-14 * 4.5x10-7 / 4.5x10-7 + 1x10-5)1/2

[H+] = 2.12x10-8 M

pH = -log(2.12x10-8) = 7.67

It will be a little greater than 7.

Hope this helps

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