Question

Calculate the molar concentration of OH− ions in an 1.37 M solution of hypobromite ion (BrO−; Kb = 4.0 10-6). M

What is the pH of this solution?

Answer 1

Let α be the dissociation of the weak base
                            BOH <---> B ⁺ + OH^-

initial conc.            c               0         0

change               -cα            +cα      +cα

Equb. conc.         c(1-α)        cα      cα

Dissociation constant, K_b = (cα x cα) / ( c(1-α)               

                                          = c α² / (1-α)

In the case of weak bases α is very small so 1-α is taken as 1

So K_b = cα²

==> α = √ ( K_b / c )

Given K_b = 4.0x10^-6

          c = concentration = 1.37 M

Plug the values we get α = 1.71x10^-3
So the concentration of [OH^-] = cα

= 1.37 x1.71x10^-3
= 2.34x 10^-3 M

Therefore the molar concentration of OH- is  2.34x 10^-3 M