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Kaplan FL 7 C/P

avalonisland888

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May 22, 2020
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I understand why C is correct. But my question is about titrations...I don't remember where, but I think there was another question in one of my practices where you dissolve an acid in a strong BASE and then titrate it with a strong base. Why would we dissolve phenylalanine in a strong ACID and then titrate it with a strong base in this question? I might be remembering this wrong, but I want to make sure.
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lumya

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Aug 7, 2018
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Phenylalanine is hydrophobic. When you dissolve it with a strong base, you deprotonate the carboxyl terminal. When you titrate that with a strong acid, you will re-protonate the carboxyl. There is only one equivalence point because strong base + strong acid = pH~7 which is what the pI of the amino acid is. The opposite is true for dissolving it in strong acid. D was incorrect because you can't get two equivalence points.
 

avalonisland888

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May 22, 2020
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Phenylalanine is hydrophobic. When you dissolve it with a strong base, you deprotonate the carboxyl terminal. When you titrate that with a strong acid, you will re-protonate the carboxyl. There is only one equivalence point because strong base + strong acid = pH~7 which is what the pI of the amino acid is. The opposite is true for dissolving it in strong acid. D was incorrect because you can't get two equivalence points.

I'm still confused as to why dissolving it in a strong base will not work. If we want to titrate a acid, do we always dissolve in acid and titrate with base? If we want to titrate a base, do we always dissolve in a base and titrate with acid?
 

lumya

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I'm still confused as to why dissolving it in a strong base will not work. If we want to titrate a acid, do we always dissolve in acid and titrate with base? If we want to titrate a base, do we always dissolve in a base and titrate with acid?
Acid or base is relevant in this question. Phenylalanine is a hydrophobic amino acid. It is neutral with a pI of approximately 7. It can be either acidic or basic depending on its environment. The reason why answer D is incorrect is because phenylalanine only has two site where protonate/deprotonation can occur, however, you will only have a one equivalence points because you are either looking at half of the titration curve.
 
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avalonisland888

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May 22, 2020
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Acid or base is relevant in this question. Phenylalanine is a hydrophobic amino acid. It is neutral with a pI of approximately 7. It can be either acidic or basic depending on its environment. The reason why answer D is incorrect is because phenylalanine only has two site where protonate/deprotonation can occur, however, you will only have a one equivalence points because you are either looking at half of the titration curve.
Did you mean "acid or base is IRrelevant"? I understand why D is incorrect and why C is correct based on the equivalence points, but I'm just trying to clarify the wording of choice C and why they are dissolving specifically in a strong acid.
 

lumya

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Did you mean "acid or base is IRrelevant"? I understand why D is incorrect and why C is correct based on the equivalence points, but I'm just trying to clarify the wording of choice C and why they are dissolving specifically in a strong acid.
Yes, sorry, I meant irrelevant. Phenylalanine is zwitterion so it doesn't matter if it's being dissolved in acid or base as it has both basic and acid areas.
 
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avalonisland888

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May 22, 2020
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Yes, sorry, I meant irrelevant. Phenylalanine is zwitterion so it doesn't matter if it's being dissolved in acid or base as it has both basic and acid areas.
Ah I see. Thank you!! Let's say the amino acid is acidic or basic and not nonpolar/polar...in that case, we can also dissolve in either strong base or acid right? And then titrate using the opposite e.g. if we have glutamic acid, we can dissolve in strong acid and then titrate with strong base?
 

lumya

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Ah I see. Thank you!! Let's say the amino acid is acidic or basic and not nonpolar/polar...in that case, we can also dissolve in either strong base or acid right? And then titrate using the opposite e.g. if we have glutamic acid, we can dissolve in strong acid and then titrate with strong base?
Yes. You can use either method. How many equivalence points will depend on the pI of the amino acid.
 
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