Question

Be sure to answer all parts. The pH of a 0.30 M solution of a weak base is 10.62. What is the Kb of the base? Kb = × 10 Enter your answer in scientific notation.

Answer 1

we have below equation to be used:

pH = -log [H+]

10.62 = -log [H+]

log [H+] = -10.62

[H+] = 10^(-10.62)

[H+] = 2.399*10^-11 M

we have below equation to be used:

[OH-] = Kw/[H+]

Kw is dissociation constant of water whose value is 1.0*10^-14 at 25 oC

[OH-] = (1.0*10^-14)/[H+]

[OH-] = (1.0*10^-14)/(2.399*10^-11)

[OH-] = 4.169*10^-4 M

Lets write the dissociation equation of B

B +H2O -----> BH+ + OH-

0.3 0 0

0.3-x x x

Kb = [BH+][OH-]/[B]

Kb = x*x/(c-x)

Kb = 4.169*10^-4*4.169*10^-4/(0.3-4.169*10^-4)

Kb = 5.8*10^-7

Answer: 5.8*10^-7