Question

The pH of a water is measured to be 7.0. The concentration of bicarbonate was measured to be 1.3*10^-3 M. What are the concentrations of carbonate, carbonic acid, and CT (total carbonates)? Assume this system is closed to the atmosphere and the temperature is 25 C.

Answer 1

for H2CO3 :

Ka1 = 4.5 × 10⁻⁷

Kb1 = 2.22 x 10^-8 M

Ka2 = 4.7 × 10⁻¹¹

HCO3- ---------------> CO32- + H+

1.3 x10^-3                       0            0

1.3x10^-3 - x                    x            x

Ka2 = [CO32-][H+] / [HCO3-]

4.7 × 10⁻¹¹ = x^2 / (1.3x 10^-3 - x)

x = 2.47 x 10^-7

[HCO3-] = 1.3 x 10^-3

concentration of carbonate [CO32-] = 2.47 x 10^-7 M

HCO3- + H2O ---------------> H2CO3 + OH-

1.3x10^-3                                    0             0

1.3x10^-3-x                                 x               x

Kb1 = x^2 / 1.3x10^-3 - x

2.22 x 10^-8 = x^2 / 1.3x10^-3 - x

x = 5.36 x 10^-6

[H2CO3] = 5.36 x 10^-6 M

concentration of carbonic acid = [H2CO3] = 5.36 x 10^-6 M

total carbonates = [HCO3-] + [CO32-] = 1.3 x 10^-3 + 2.47 x 10^-7

                           = 1.3 x 10^-3 M