Help please..... Calculating Hydroxide ion (OH-) concentration.?

Question

An aqueous solution contains .050 M of methylamine. Kb for methylamine is 4.4 x 10^ -4. What is the concentration of OH-?

I keep getting the wrong answer, please help.

Thanking you in advance.

Answer 1

What's Kb?

Kb = [HA+][OH-]/[A-].  What's the relationship?  We know Kb.  We know the molar concentration, 0.05M.  We know the molar relationship is one to one.  

So, to solve, Kb = [x][x]/[CH3NH2]

Define [CH3NH2] as [0.05 - x].

Resolve as Kb = x^2/0.05. (we assumed x was negligible when subtracted from the initial concentration of the base)