Question

Consider a 0.64 M solution of dimethylamine, (CH₃)₂NH (K_b = 5.4×10^-4).

Mark the major species in the solution.

_______TrueFalse (CH₃)₂NH
_______TrueFalse (CH₃)₂NH₂⁺
_______TrueFalse H₂O
_______TrueFalse H⁺
_______TrueFalse OH^-

Complete the following table (in terms of "x", the amount of dimethylamine which reacts). Minus signs must be included, omit positive signs and omit molarity units (they are assumed).

	(CH3)2NH	(CH3)2NH2+	OH-
Initial
Change
Equilibrium	0.64 - x

Determine the equilibrium concentration of (CH₃)₂NH₂⁺
M

Calculate the pH of the solution.
pH =

Answer 1

(CH3)2NH   ----------------> (CH3)2NH2+   +    OH-

0.64                                          0                  0        ---------> initial

- x                                            +x                 +x         ---------> change

0.64 - x                                       x                    x          ---------> equilibrium

Kb = [(CH3)2NH2+][OH-] / [(CH32NH)]

5.4 x 10^-4 = x^2 / 0.64 - x

x = 0.0183

equilibrium concentration of (CH3)2NH2+ = 0.018 M

[OH-] = 0.0183 M

pOH = - log (0.0183) = 1.74

pH = 12.26

Major species :

OH- , (CH3)2NH2+ , (CH3)2NH