Question

Carbon dioxide dissolves in water to form carbonic acid, which is primarily dissolved CO2. Dissolved CO2 satisfies the equilibrium equation

CO_2(gas)<------>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 CO2. For a CO2 partial pressure of 3.1×10-4 atm in the atmosphere, what is the pH of water in equilibrium with the atmosphere?

Answer 1

Given data,

P_CO2 = 3.14 x 10^-4 atm

Concentration of CO2 = Kco2 x Pco2

= 0.032 x 3.1 x 10^-4

= 9.92 x 10^-6 M

Ka = [H+]^2 / [CO2(aq)]

4.46 x 10^-7 = [H+]^2 / 9.92 x 10^-6

[H+]^2 = 4.46 x 10^-7 x 9.92 x 10^-6

= 4.42 x 10^-10

[H+] = 2.10 x 10^-5 M

pH = -log[H+]

= -log ( 2.10 x 10^-5)

= 4.67