Hey!
I just wanted to pose a question that I hope someone can find the time to answer.
It's regarding part of my homework - don't worry, I already have the answer. I was just confused about some of the methods.
The question is part of a larger one:
"Calculate the pOH of a 0.0500 mol dm-3 aqueous solution of methylamine. State any assumptions made in your calculation."
We are given the Kb value at 4.37*10-4 mol dm-3
The assumption is referring to the fact that I can disregard the concentration value in the denominator of this equation [H+]2/C-[H+]=Ka
I could see two ways of calculating the answer. The first one gave me the right answer.
If [H+]2/C=Ka then [OH-]2/C=K
We can isolate for the [OH-] value (0.004674), and take the negative logarithm in base 10 of it, producing a pOH of 2.33
I wanted to be thorough and double check my answer, so I ran through the next method too:
pOH = 14 - pH
pH = -log[H+]
[H+] = sqrt(Ka*C)
Ka = 10-pKa
pKa = 14 - pKb
pKb = -log(Kb)
Putting in values and collecting equations I get: pOH = 14 - (-log(sqrt(10-(14-(-log0.000437))*0.05)))
This gives me a pOH value of 8.03 which is ridiculous, since methylamine is a weak base.
My question: what am I doing wrong in the second method? I know it's an unnecessary way around it, but from what I've read all the constants are mathematically connected to their conjugates. In theory, shouldn't it work?
Thank you!
