Question

What is the hydroxide-ion concentration in a 0.22 M solution of Na2CO3? For carbonic acid, Ka1 = 4.2  10–7 and Ka2 = 4.8  10–11

Answer 1

CO3^2- + H2O --------> HCO3^- + OH^-

       I                0.22                           0             0

      C               -x                              +x            +x

      E              0.22-x                          +x             +x

                    Kb    = Kw/Ka2

                             = 1*10^-14/4.8*10^-11

                             = 2.1*10^-4

               Kb    = [HCO3^-][OH-]/[CO3^2-]

   2.1*10^-4      = x*x/0.22-x

        2.1*10^-4 *(0.22-x) = x²

             x   = 0.0067

     [OH-] = x = 0.0067M