Question

A 0.025 M solution of hydroxylamine has a pH of 9.11. What is the value of Kb for this weak base?

H2?NOH(aq)+H2O(?)?H3?NOH+(aq)+OH-(aq)

Answer 1

H2NOH(aq)   + H2O(l)-------------->H3NOH+(aq)     + OH-(aq)

0.025                                                    0                        0 --------------------initial

0.025-x                                                   x                      x -------------------equilibrium

Kb= [H3NOH+][OH-]/[H2NOH]

Kb= x^2/(0.025-x ) -------------------------------(1)

we know pH= 9.11

pH+ pOH = 14

pOH= 14-pH= 14-9.11=4.89

pOH= -log[OH-]

[OH-]= 10^-pOH= 10^-4.89 = 1.28 x 10^-5

[OH-]= x = 1.28 x 10^-5

from equation (1)

Kb= x^2/(0.025-x ) = (1.28 x 10^-5)^2/0.025

   Kb = 6.63 x 10^-9