Question

The solubility of CaCO3 is pH dependent.

a. Using the Ksp for CaCO3 and the Kb value for CO3-2, determine the equilibrium constant for the reaction below. CaCO3(s) + H2O(l) <-> Ca2+(aq) + HCO3-(aq) + OH-

b. If we assume the only sources of Ca2+, HCO3-, and OH- ions are from the dissolution of CaCO3, what is the molar solubility expression from part a, and what is the pH?

c. If there is another source of HCO3- (i.e., from the dissolution of CO2), the pH will decrease. What is the molar solubility of CaCO3at the pH of the ocean (pH = 8.3)?

Answer 1

CaCO₃ +H₂O Ca²⁺ + HCO₃^- +OH^-

equilibrium constant of the reaction

K_eq = (1) as, [CaCO₃] and [H₂O] is constant]

ksp =

Kb =

or, Ksp/Kb =   (2)

again, kw for water  =

or , 1/[H⁺] = [OH^-]/ k_w

Putting the value equation 2

Ksp/Kb =

Comparing with equation 1

Ksp/Kb = k_eq/K_w

or, k_eq = ksp k_w/ k_b = 3.3610^-910^-14/ 2.110^-4 = 1.610^-19