Question

Dimethylamine, (CH3)2NH, is commonly found in plants and food (in very small amounts). A solution of 0.100 M dimethylamine is 2.4% ionized. Calculate Kb for this base.

Answer 1

(CH₃)₂NH + H₂O (CH₃)₂NH₂⁺ + OH^-

Let concentration of (CH₃)₂NH = c

degree of dissociation = x

Lets write (CH₃)₂NH as B and (CH₃)₂NH₂⁺ as BH⁺

ICE table is

	B	BH+	OH-
Initial	c	0	0
Change	-cx	+cx	+cx
Equilibrium	c - cx	cx	cx

Now, base dissociation constant

Kb =

Or, Kb =

Or, Kb =

Given, C = 0.100 M.

Percent ionization = 2.4%

So, x = (2.4/100) = 0.024

Then, Kb = [0.100× (0.024)²] /(1 - 0.024)]

= (5.76×10^-5/0.976)

= 5.90× 10^-5 .