Sapling Week 2 #5

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Sapling Week 2 #5

Postby OwenSumter_2F » Wed Jan 13, 2021 1:58 pm

I'm having problems with Sapling Week 2 #5. The question is:

The Kb for an amine is 2.666×10−5. What percentage of the amine is protonated if the pH of a solution of the amine is 9.976 ? Assume that all OH− came from the reaction of B with H2O.

I tried to work backwards by finding the pOH from the pH then finding [OH-] by doing the inverse of -log. Then, I used that value as x, and worked out [B] with the Kb value. Finally I did the percentage protonated formula but my answer was said to be incorrect. Is there a step I'm missing or doing incorrectly?

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Re: Sapling Week 2 #5

Postby RDove_3G » Wed Jan 13, 2021 2:41 pm

I think the way you described calculates the ratio of protonated amine to amine at equilibrium, but the question is asking for the percent of the amine that was protonated which would be protonated amine at equilibrium over the initial concentration of amine. To do this you have to first solve for the protonated amine at equilibrium which you know will be equal to the OH- concentration because they are at equal molar ratios. Then you can plug that in to the Kb expression to find the amount of base amine at equilibrium. The initial concentration of the amine is the sum of the equilibrium amines (protonated and not). Then you can find the percent. Hope this helps!

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Re: Sapling Week 2 #5

Postby Claire_Latendresse_1E » Wed Jan 13, 2021 2:42 pm

You're correct that you would need to find the pOH from the pH and do 10^-pOH to find [OH-]. You use that value as x and plug it into the Kb equation to find [B]. In this case, [B] = x^2 / Kb. For the percent, you use [BH3+] / ([BH] + [B]). It seems like you did the problem correctly, so there's probably just an algebra error somewhere along the way. I made a lot of mistakes lol.

Adalia 3E
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Re: Sapling Week 2 #5

Postby Adalia 3E » Wed Jan 13, 2021 10:32 pm

I kept getting this problem wrong because in the final step, I kept calculating [BH+]/[B] rather than [BH+]/([BH+][B]) like what Claire specified. That might be a place that made you get the incorrect answer

Mary Gallo 1G
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Re: Sapling Week 2 #5

Postby Mary Gallo 1G » Wed Jan 13, 2021 10:42 pm

Hi! Could someone explain why we calculate [BH+]/([BH+][B]) instead [BH+]/[B] for the last step? I'm trying to understand it conceptually. Thank you!

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