Achieve Question 5

[Nolan Reber 3F]

For this question on achieve:
The Kb for an amine is 5.789×10−5. What percentage of the amine is protonated if the pH of a solution of the amine is 9.653? Assume that all OH− came from the reaction of B with H2O.

I went through the problem, finding Ka and the [H+} concentration, and got 43.7%.

Did I just mix up protonation and deprotonation?

[CarolYassa2K]

For this question, you first need to find pOH to find the concentration of [OH-]. To do this, you raise 10^log[OH-]. Then you plug in the value for [OH-] into Kb=[BH+][OH-] / [B].
[OH-]^2/ initial M = Kb use this to find the initial M.
% protonated = [BH+] / [B] formal x 100

[Vishwa Diwan 1D]

I was following those steps and found the pOH to be 4.727 (my pH was 9.273), and therefore found the OH- concentration to be 0.000019 after doing 10^-(4.727). But once I set my Kb equal to (0.000019)^2/[B]-------Kb was 5.416E-5, I got the initial concentration of B to be 0.000006. Not sure what I am doing wrong because that makes the percent protonation extremely large if I plug it back into the [BH] over [B]. Is there a step that I am missing or doing wrong?

[Hannah Mirmo 1F]

I was following those steps and found the pOH to be 4.727 (my pH was 9.273), and therefore found the OH- concentration to be 0.000019 after doing 10^-(4.727). But once I set my Kb equal to (0.000019)^2/[B]-------Kb was 5.416E-5, I got the initial concentration of B to be 0.000006. Not sure what I am doing wrong because that makes the percent protonation extremely large if I plug it back into the [BH] over [B]. Is there a step that I am missing or doing wrong?

Hi, when you plug it in, make sure to do [BH+] / [BH+]+[B] then multiply by 100%. From looking at your steps that seems like the only error possible.