Achieve #5, week 2

[almaortega]

Can someone please explain the process of this problem after finding the concentration of [OH-]? I got super confused on the ICE table and moving forward. Thank you

[Irene Kang 3F]

You have to use pOH and find the concentration of OH-. This will be the x value for when you plug it in to the Kb formula. Then, you have to solve for the original concentration of B with the values of x and Kb. To find the percent protonated, divide the x value by the original concentration.

[Alice Guey 1B]

For this problem, I made the assumption that the pH was measured when the reaction is at equilibrium. Therefore, [OH-] corresponds to the value of x you would usually input in an ICE box. From there, you know that [OH-]=[BH+] in which B represents the amine. The only value you do not know is the initial concentration of B, [B]. You can calculate [B] by setting up the equation Kb=[OH-][BH+]/[B]-[OH-]. Finally, you can calculate the percentage protonated by setting up the equation percentage protonated=([BH+]/[B])x100. Hope this helps!

[Amy Huynh 1B]

After you find the value of [OH-], you have also found the value of [BH+] because the concentrations would be equal under the assumption that "all OH− came from the reaction of B with H2O". Then, you would want to write out the expression for Kb, referring to the balanced chemical equation. We have the value of Kb, [BH+] and [OH-]. Therefore, we can solve for the value of [B]. Once we have the [B], we can use that to find the percentage of protonation by using the formula [BH+]/[B]formal.
*[B]formal is the initial base concentration [B] added to the the concentration of protonated species [BH+].