Question

Which of the following occurs when HCl is added to a budder containing (CH₃)₃N and (CH₃)₃NH⁺?  

		The concentraion of (CH3)3N will increase.
		The concentraion of (CH3)3NH+ will decrease.
		The concentraion of (CH3)3N will decrease.
		The concentraion of (CH3)3NH+ will increase

Of the following, which is the best base to create a buffer with a pH = 8.0?

		Ethylamine, Kb = 6.4 x 10-4
		Aniline, Kb = 4.3 x 10-10
		Hydrazine, Kb = 1.3 x 10-6
		Pyridine, Kb = 1.7 x 10-9

Answer 1

1.

(CH₃)₃NH+ + H₂O <----> (CH₃)₃N + H₃O+

H₃O+ can be represented by H+

Hence additino of HCl adds more H+ ions. To keep the equilibrium constant same, the backward reaction has to take place. This leads to an increase in concentration of (CH₃)₃NH+ .

2. Kb for the reactions

ethyl amine

C₂H₅NH₂ + H₂O ⇄ C₂H₅NH₃⁺ + OH^-

Aniline

C₆H₅NH₂ + H₂O ⇄ C₆H₅NH₃⁺ + OH^-

Hydrazine

N₂H₄ + H₂O ⇄ N₂H₅⁺ + OH^-

Pyridines

C₅H₅N + H₂O ⇄ C₅H₅NH⁺ + OH^-

Kb for the above reactions can be writtens as

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

B indicates the base

pH=8

POH= 14-8=6

[OH-]= 10^(-6)

For all the bases this remins the same

since the numerator in the Kb equation remains the sane

Kb= (10^-6)²/ X

whee X is the base concentration

X will be lowest when Kb is highest.

Kb is highest for Ethylaine and hence lower concentrations of ethylamine does the job for creating a buffer of pH=8