Carbon dioxide dissolves in water to form carbonic acid, which is primarily diss

Question

Carbon dioxide dissolves in water to form carbonic acid, which is primarily dissolved CO2. Dissolved CO2 satisfies the equilibrim equation: CO2(g) <---> CO2(aq) K=0.032. The acid dissociation constants listed in most standard reference texts for carbonic acid actually apply to dissolved CO2. For a CO2 partial pressure of 8.6x10^-4 bar in the atmosphere, what is the pH of water in equilibrium with the atmosphere? (For carbonic acid Ka1= 4.46x10^-7 and Ka2= 4.69x10^-11).

Hint: Since carbonic acid is primarily dissolved CO2, the concentration of H2CO3 must be taken as equal to that of dissolved CO2.

Explanation / Answer

P[CO2(aq)] = Kco2 x P(co2(g))

                   = 0.032 x 8.5 x 10^-4 atm

                   = 2.75 x 10^-5 atm

Using the CO2 integrated equation

Ka1 = [H+][HCO3-]/[CO2(aq)]

[H+] = [HCO3-]

4.46 x 10^-7 = [H+]^2/2.75 x 10^-5

[H+] = 3.50 x 10^-6 M

pH = -log[H+] = 5.45

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