Question

A weak base B (neutral charge) is dissolved in water at a concentration of 0.500 M. The concentration of the resulting ions is 0.170. What is the numerical value of Kb? The answer should be written to 4 decimal places and correct within +/-0.0020. Don't forget that [B] is going to be less than 0.500 M.

Answer 1

                                   BOH(aq) + H2O(l) --------------> B+(aq) + OH-(aq)

          initial               0.5 M                                                0 M       0 M

          change           0.17 M                                                 0.17 M     0.17M

    equilibrium           0.5-0.17                                            0.17             0.17

          Kb= [B+][OH-]/[BOH]

           Kb = 0.17^2/(0.5-0.17) = 0.0876