Question

Consider a 6×10⁻² M  solution of the weak base methylamine.

The pKb  of methylamine is 3.34  

You can search for the structure of this compound online, although precise knowledge of the structure is not needed.

Calculate the pH and the concentration of all species present in equilibrium.

1.Calculate the concentration of   methylammonium ions.

2.Calculate the concentration of   methylamine in equilibrium.

Answer 1

Methylamine is CH₃NH₂

In aqueous solution it dissociates as

CH₃NH₂ + H₂O CH₃NH₃⁺ + OH^-

ICE table is

X is degree of dissociation

	[CH3NH2]	[CH3NH3+]	[OH-]
Initial	C	0	0
Change	- Cx	+ Cx	+Cx
Equilibrium	C-Cx	Cx	Cx

Then, Kb = [CH₃NH₃⁺] [OH^-]/[CH₃NH₂]

= [Cx*Cx/(C-Cx)]

Kb = Cx²/ (1-x)

As methyl amine is weak base , 1>>x

Kb = Cx²

Given , pKb = 3.34

Or, - logKb = 3.34

Then, Kb = 0.000457

Or, Cx² = 0.000457

Or, X² = (0.000457/C) = (0.000457/6*10^-2) = 0.0076

Or, x = 0.087

Then, pOH = - log [OH^-] = - log[Cx] = - log(6*10^-2*0.087)

= -log(0.00522) = 2.282

Then pH = 14 - pOH = 14 - 2.282 = 11.717

Concentration of methylammonium ions is [CH₃NH₃⁺] = [OH^-] = 0.00522 M

and, concentration of methylamine in equilibrium is

[CH₃NH₂] = (C - Cx) = (6*10^-2 - 0.00522) = 0.05478 M