Question

you are assigned the task of determining the identity of an unknown weak base. if a 4.70x10^-6 M solution of the weak base has a pH of 7.820, determine the kb value of the weak base.

answer = 1.08x10^-7

however i keep getting 9.29x10^-8

how do i solve

Answer 1

Answer - We are given, [B] = 4.70*10^-6 M, pH = 7.820

We know,

pH +pOH = 14

so, pOH = 14 -pH

              = 14 – 7.820

              = 6.18

So, [OH^-] = 10^-pOH

                = 10^-6.18

                = 6.61*10^-7 M

So at equilibrium , x = [OH^-] = 6.61*10^-7 M

So, Kb = [OH^-][BH⁺] / [B]

[B] = 4.70*10^-6 M – 6.61*10^-7 M

         = 4.04*10^-6 M

Kb = 6.61*10^-7 * 6.61*10^-7 / 4.04*10^-6

       = 1.08*10^-7