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Topic: Predominant Forms of an Ion at pH 7.5?  (Read 3733 times)

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Offline Simons2

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Predominant Forms of an Ion at pH 7.5?
« on: October 30, 2016, 06:40:15 PM »
Citric acid-C6H5O7 (3-)
Ethylenediamine-C2H6N2
Aniline-C6H5NH3+
Propanoic acid-CH3CH2COOH
Lactic acid-C3H6O3


The bolded answers are my guesses for the molecules.
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Offline AWK

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Re: Predominant Forms of an Ion at pH 7.5?
« Reply #1 on: October 30, 2016, 07:41:09 PM »
Why you decided in such a way (3 decisions are wrong)?
Do not guess, substantiate decision with pKa.
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Offline Simons2

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Re: Predominant Forms of an Ion at pH 7.5?
« Reply #2 on: October 30, 2016, 07:52:29 PM »
Quote from: AWK on October 30, 2016, 07:41:09 PM
Why you decided in such a way (3 decisions are wrong)?
Do not guess, substantiate decision with pKa.

I used the term guess as a figure of speech.... I actually looked up the pka's on the molecules and determined that way. There are some molecules where there are no pKa's that I could find.
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Offline AWK

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Re: Predominant Forms of an Ion at pH 7.5?
« Reply #3 on: October 30, 2016, 08:13:58 PM »
According to our rules:
http://www.chemicalforums.com/index.php?topic=65859.0
you should show your work, at least you reasoning, but not guesses.

My guess - all acids are predominatly ionized at pH 7.5, all bases - not-ionized - is much better than yours (I do not say - ideally correct).
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Offline Simons2

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Re: Predominant Forms of an Ion at pH 7.5?
« Reply #4 on: October 30, 2016, 08:17:24 PM »
Quote from: AWK on October 30, 2016, 08:13:58 PM
According to our rules:
http://www.chemicalforums.com/index.php?topic=65859.0
you should show your work, at least you reasoning, but not guesses.

My guess - all acids are predominatly ionized at pH 7.5, all bases - not-ionized - is much better than your guess (I do not say - ideally correct).

As I said earlier, the term "guess" was just a figure of speech that I use often. I didn't take the time to write down all the pKa's- true. And after looking over it again, I believe I got 4 out of the 5 wrong not 3. All are corrected now.
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Offline AWK

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Re: Predominant Forms of an Ion at pH 7.5?
« Reply #5 on: October 30, 2016, 11:10:24 PM »
You are right - 4 of 5. My crude rule gives only one error.

In the case of ethylenediamine, different sources give different values for both pKas values. Then you may get predominant form with 1 or 2 protons.
With your answer give values of pKa and source of data. For other compounds there are no ambiguities.
Many universities compiled their own tables - then use them without comment.
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Offline Babcock_Hall

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Re: Predominant Forms of an Ion at pH 7.5?
« Reply #6 on: October 31, 2016, 09:36:25 AM »
Simons2,

I am not sure why you could not find pKa values for lactic or propanoic acids.  They are common substances, and can be found in some organic chemistry and biochemistry textbooks.  There are many reliable pKa tables available.  Explaining your reasoning with respect to a compound for which you did find a value is not a bad exercise.
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