Question

What is the pH at 25 ∘C of water saturated with CO2 at a partial pressure of 1.20 atm ? The Henry's law constant for CO2 at 25 ∘C is 3.1×10−2mol/L-atm. The CO2 is an acidic oxide, reacting with H2O to form H2CO3.

Answer 1

C = Kh/P   as per Henry law   where C = Molarity of CO2 dissolved , P = partial pressure = 1.2 atm ,

Kh is henry cosntant = 3.1 x 10^-2 mol/Latm

C = ( 3.1 x 10^-2 mol/Latm) / ( 1.2 atm)

= 0.0258 M

CO2 combines with H2O to form H2CO3 , hence H2CO3 Molarity = 0.0258 M

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

Initial              0.0258                     0                     0

at equilibrium   0.0258-X                    X                  X

Ka = [H+] [HCO3-] /[H2CO3]

5 x 10^-7 = X^2 / ( 0.0258-X)    here we approximate 0.0258-X as 0.0258 sicne X value is small as Ka is small

X = [H+] = 1.137 x 10^-4

pH = -log ( 1.137 x 10^-4) = 3.94