Question

A) In the laboratory, a general chemistry student measured the pH of a 0.566 M aqueous solution of triethylamine, (C₂H₅)₃N to be 12.221.  

Use the information she obtained to determine the K_b for this base.  

K_b(experiment) = ___

B) In the laboratory, a general chemistry student measured the pH of a 0.566 M aqueous solution of quinoline, C₉H₇N to be 9.262.

Use the information she obtained to determine the K_b for this base.  

K_b(experiment) = ___

Answer 1

A)

use:

pH = -log [H+]

12.22 = -log [H+]

[H+] = 6.012*10^-13 M

use:

[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)/(6.012*10^-13)

[OH-] = 1.663*10^-2 M

(C2H5)3N dissociates as:

(C2H5)3N +H2O -----> (C2H5)3NH+ + OH-

0.566 0 0

0.566-x x x

Kb = [(C2H5)3NH+][OH-]/[(C2H5)3N]

Kb = x*x/(c-x)

Kb = 1.663*10^-2*1.663*10^-2/(0.566-1.663*10^-2)

Kb = 5.037*10^-4

Answer: 5.04*10^-4

B)

use:

pH = -log [H+]

9.262 = -log [H+]

[H+] = 5.47*10^-10 M

use:

[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)/(5.47*10^-10)

[OH-] = 1.828*10^-5 M

C9H7N dissociates as:

C9H7N +H2O -----> C9H7NH+ + OH-

0.566 0 0

0.566-x x x

Kb = [C9H7NH+][OH-]/[C9H7N]

Kb = x*x/(c-x)

Kb = 1.828*10^-5*1.828*10^-5/(0.566-1.828*10^-5)

Kb = 5.905*10^-10

Answer: 5.91*10^-10