Question

Assuming a concentration of 1200 ppmv, What will be the pH of rainwater assuming CO2 is the only compound influencing pH?

You can ignore carbonate in this question.

Answer 1

Partial pressure of CO2 in air = 1 atm x 1200 x 10^-6

= 0.0012 atm

Concentration of H2CO3 = K_H x P_CO2

= 3.4*10^-2 M/atm x 0.0012 atm

= 0.0000408 M

The dissociation reaction

H2CO3 = H+ + HCO3-

CO2 will not be depleted from the air and H2CO3 will be constant

[H+] = [HCO3-]

first acid dissociation expression of the reaction

Ka = [H+] [HCO3-] / [H2CO3]

2 x 10^-4 = [H+]²/0.0000408

[H+]² = 0.0000408 x 2 x 10^-4 = 8.16 x 10^-9

[H+] = 9.033 x 10^-5

pH = - log [H+] = - log [9.033 x 10^-5]

pH = 4.04