Question

What would be the pH of a carbonated soda at room temperature made by saturating pure water with pure CO2 if the pressure in the container was at 2 atm. pressure? Would the pH increase, decrease or stay the same once the soda was cooled to 4 degrees C? Explain.

Answer 1

The Carbonated soda means CO2 saturated in water and solubility of CO2 in water at room temperature is 0.03 moles/L or 0.03 M. CO2 in water present as Carbonic acid, which can dissociate as follows:

CO2 + H2O --> H2CO3 --> H+ + (HCO3)-

The Ka for this is 4.2 × 10-7
The second dissociation ,
HCO3- → H+ + CO3 2- has Ka = 4.8 × 10-11 and

Due to very small values of this ,it can be ignored.

Now calculate [H+] in 0.03 M solution of H2CO3 using the Ka equation:

Ka = [H+]² / [H2CO3]
4.2*10^-7 = [H+]² / 0.03
[H+]² = (4.2*10^-7)*( 0.03)
[H+]² = 1.26*10^-8
[H+] = 1.12*10^-4 M

Now pH of this water:

pH = -log [H+]
pH = -log (1.12*10^-4)
pH = 3.95

the pH would be increase, when the soda was cooled to 4 degrees C because the solubility of CO2 or any gas increase with decreasing temperature.