Question

Find the [OH-] and pH of a 0.66 M methylamine solution

Answer 1

Find the [OH-] and pH of a 0.66 M methylamine solution

Methylamine solution concentration is 0.66 M and methylamine base constant (kb) = 4.4x10^-4.


CH₃NH₂ + H₂O -> [OH^-] + [CH3NH3⁺]
0.66 M____0______0________0______(before reaction; x: production concentration)
___x_____________ x________x______(reacting with H₂O is not considered a reactant)
(0.66- x)___________-x_______-x______(after reaction)

Note: we don't count H₂O on calculation.

Formula of Kb = [CH₃NH₃⁺][OH^-]/[CH₃NH₂]
-> 4.4*10^-4 = x²/(0.66-x)

(2.904*10^-4) – (4.4 *10^-4) x = x²

x² + (4.4 *10^-4) x - (2.904*10^-4) = 0

If we solve this quadratic equation we will get x = -0.0172 or x= 0.0168

Negative value is not possible so x= 0.0168

[OH^-] = 0.0168 M

pOH = -log [OH^-] = -log (0.0168) = 1.77