Question

CO2 dissolves in water to produce H2CO3 in the same way that SO2 produces H2SO3. (a) Confirm by calculation that the pH of CO2-Saturated water at 25 degrees C is 5.6, given that the CO2 concentration in air is 365ppm. For carbon dioxide, the henry's law constant KH=3.4x10^-2M atm^-1 at 25 degrees C. The Ka for carbonic acid, H2CO3, is 4.5x10^-7. (b) Recalculate the pH for carbon dioxide concentration of 560ppm.

Answer 1

a)

CO₂ (g) + H₂O (l) ? H₂CO₃ (aq)

K_H = [H₂CO₃ (aq)]/P_CO2 = 0.034 M/atm

so [H₂CO₃ (aq)] = (0.034 M/atm)(365 x 10^-6 atm) = 1.24 x 10^-5 M

H2CO3(aq) ----> H+ (aq) + HCO3-(aq)

K_a1 = 4.5 x 10^-7 = [H⁺][HCO₃^-]/[H₂CO₃ (aq)] = [H⁺]²/[H₂CO₃ (aq)]

or [H⁺] = [(4.5 x 10^-7)(1.24 x 10^-5)]^0.5 = 2.36 x 10^-6 M or pH = 5.6

b) Recalculate the pH for a carbon dioxide concentration of 560 ppm, i.e., double that of the preindustrial age.

[H₂CO₃ (aq)] = (0.034 M/atm)(560 x 10^-6 atm) = 1.90 x 10^-5 M

[H⁺] = [(4.5 x 10^-7)(1.33 x 10^-5)]^0.5 = 2.93 x 10^-6 M or pH = 5.53