Question

In the laboratory, a general chemistry student measured the pH of a 0.344 M aqueous solution of hydroxylamine, NH2OH to be 9.762. Use the information she obtained to determine the Kb for this base.

In the laboratory, a general chemistry student measured the pH of a 0.344 M aqueous solution of dimethylamine, (CH₃)₂NHto be 12.138.  
Use the information she obtained to determine the K_b for this base.  

Answer 1

use:
pH = -log [H3O+]
12.14 = -log [H3O+]
[H3O+] = 7.278*10^-13 M

use:
[OH-] = (1.0*10^-14)/[H3O+]
[OH-] = (1.0*10^-14)/(7.278*10^-13)
[OH-] = 1.374*10^-2 M
(CH3)2NH dissociates as:

(CH3)2NH +H2O     ----->     (CH3)2NH2+   +   OH-
0.344                   0         0
0.344-x                 x         x


Kb = [(CH3)2NH2+][OH-]/[(CH3)2NH]
Kb = x*x/(c-x)
Kb = 1.374*10^-2*1.374*10^-2/(0.344-1.374*10^-2)
Kb = 5.72*10^-4

Answer: 5.72*10^-4