Respuesta :
Since methylamine (CH3NH2) is a weak base, the major species in solution are CH3NH2 and H2O.
Both are bases; however, since water can be neglected as a source of OH-, the dominant equilibrium is:
CH3NH2(aq) + H2O(l) = CH3NH3+(aq) + OH-(aq)
and
The concentrations are as follows:
[OH-] = [CH3NH3+] = x
[CH3NH2] = 1.0 - x
Then,
Assume that 1 >> x, then:
(1.0 - x) = 1.0
Kb = x2 / 1.0
x2 = Kb
x = (Kb)0.5 = (4.38×10-4)0.5 = 0.0209
x= [OH-] = 0.0209 M
We can convert between [OH-] and pOH using the following equations:
pOH = - log[OH-]
pOH = - log(0.0209) = 1.68
pOH = 1.68
For any aqueous solution at 25∘C:
pH + pOH = 14
pH = 14 - pOH = 14 - 1.68 = 12.32
pH = 12.32
Answer: pH = 12.32
Both are bases; however, since water can be neglected as a source of OH-, the dominant equilibrium is:
CH3NH2(aq) + H2O(l) = CH3NH3+(aq) + OH-(aq)
and
The concentrations are as follows:
[OH-] = [CH3NH3+] = x
[CH3NH2] = 1.0 - x
Then,
Assume that 1 >> x, then:
(1.0 - x) = 1.0
Kb = x2 / 1.0
x2 = Kb
x = (Kb)0.5 = (4.38×10-4)0.5 = 0.0209
x= [OH-] = 0.0209 M
We can convert between [OH-] and pOH using the following equations:
pOH = - log[OH-]
pOH = - log(0.0209) = 1.68
pOH = 1.68
For any aqueous solution at 25∘C:
pH + pOH = 14
pH = 14 - pOH = 14 - 1.68 = 12.32
pH = 12.32
Answer: pH = 12.32
