Question

Carbon dioxide dissolves in water to form carbonic acid, which is primarily dissolved CO_2. Dissolved CO_2 satisfies the equilibrium equation CO_2(g) CO_2(aq) K=0.032 M atm^-1 The acid dissociation constants listed in most standard reference texts for carbonic acid actually apply to dissolved CO_2. For a CO_2 partial pressure of 7.1x10^-4 atm in the atmosphere, what is the pH of water in equilibrium with the atmosphere?

Answer 1

The concentration of carbonic acid is proportional to partial pressure of CO2

[H2CO3] = K X pCO2 = 0.032 X 7.1 X 10^-4 = 2.272 X 10^-5 M

Now dissociation constant of carbonic acid H2CO3 <--> H+ (aq) + HCO3- (aq), Ka = 2.5 x 10^-4

So [H+] = (Ka X [H2CO3] )^1/2
[H+] = (2.5 X 10^-4 X [2.272 X 10^-5] )^{1/2 =} 0.753 X 10^-4
pH = -log [H+] = 4.123