[domique]

Hi!
 
What is the method to use in order to solve these problems?

1) Acridine has pkaH=5.6. At pH= 8.6 what state is acridine present in ?
2) Imidazole has pkaH= 7.4. At pH=7 it is present in neutral state , anionic state, cationic state or none?
3)  p-hydroxybezoic acid has pka (COOH)= 4.1 e pka (OH)= 8.9 .At pH= 7  it is present in neutral state, mono-anionic state, di-anionic state or none?

[Arkcon]

OK.  Let's get started.  What is the definition of pKa?  And how is it determined?

[domique]

OK.  Let's get started.  What is the definition of pKa?  And how is it determined?

 pKa is the negative base-10 logarithm of the acid dissociation constant of a solution.

pKa = -log10Ka

The lower the pKa value, the stronger the acid, therefore here are my answers to the questions ( not sure if it's right)
1) acridine has an aromatic amine which at pH=8.6 should be protonated ?
2)imidazole would be partially protonated ( cationic form ?)
3)p-hydroxybenzoic acid should be in di-anionic state ?

[Arkcon]

OK.  Let's get started.  What is the definition of pKa?  And how is it determined?

 pKa is the negative base-10 logarithm of the acid dissociation constant of a solution.

pKa = -log10Ka

The lower the pKa value, the stronger the acid, therefore here are my answers to the questions ( not sure if it's right)

Hrm.  That is the text book definition all right.  I was looking for something else ... maybe ... how do we use pKa?  Meh.  don't worry about it for now, because ...

1) acridine has an aromatic amine which at pH=8.6 should be protonated ?
2)imidazole would be partially protonated ( cationic form ?)
3)p-hydroxybenzoic acid should be in di-anionic state ?

Now lets try to look at these critically.  For the acridine solution, the pH is high, and it has a low pKa, so you're correct, it will be protonated.

The imidiazole is a tough one, the pH is right at the pKa.  Still, is it in an environment were there is more than enough H+, or not.  Partially protonated, isn't applicable here -- there's only one proton, its either on or off.

 p-hydroxybezoic acid has two pKa's and the given pH is right in the middle of them.  Is it correct for both protons be the same?

[domique]

2) imidazole should be in neutral state, then
3) p-hydroxybezoic acid should be in mono-anionic state ?

[Babcock_Hall]

I think that it is misleading to say that imidazole has a pK_a of about 7.  The imidazolium cation has this pK_a.  There is a second pK_a, but it is much higher (neutral imidazole would be the acid and an anion would be the conjugate base for this second loss of a proton).  Likewise for acridine, except there is not second pK_a of which I am aware.  I would use the Henderson-Hasselbalch equation for a somewhat more formal answer to each problem; however, I believe in reasoning out the problem in a qualitative fashion first.

[clarkstill]

Didn't they say pKaH?

[domique]

Didn't they say pKaH?

Yes, it's pkaH, but I'm still not sure about the answers of problem 2 and 3...

[Babcock_Hall]

With respect to 2) I suggest that you draw out the structures, so that we can be sure that we are discussing the same thing.  This might be useful for 3) also.

[domique]

With respect to 2) I suggest that you draw out the structures, so that we can be sure that we are discussing the same thing.  This might be useful for 3) also.

[domique]

Here are the chemical structures...

[Babcock_Hall]

Both structures look fine.  Here is the Henderson-Hasselbalch equation:  pH = pK_a + log₁₀{[conj. base]/[conj. acid]}.  When the ratio of  conjugate base to conjugate acid is less than one, the logarithm will be a negative number.  When the ratio is greater than one, the logarithm will be positive.  So with respect to 2) we know that the pH is 7 and the pK_a is 7.4.  Qualitatively, what does this tell us about the ratio of base to acid?

[Babcock_Hall]

Didn't they say pKaH?

I don't recall having seen this symbol before, but I take it that it means the conjugate acid of the chemical in question.  If so, then I withdraw my objection.

[domique]

Both structures look fine.  Here is the Henderson-Hasselbalch equation:  pH = pK_a + log₁₀{[conj. base]/[conj. acid]}.  When the ratio of  conjugate base to conjugate acid is less than one, the logarithm will be a negative number.  When the ratio is greater than one, the logarithm will be positive.  So with respect to 2) we know that the pH is 7 and the pK_a is 7.4.  Qualitatively, what does this tell us about the ratio of base to acid?

Ok,I got it...in regards to problem no.2 imidazole is in cationic state and in case of the p-hydroxybenzoic, it is in mono-anionic state 

[Babcock_Hall]

I cannot make out your most recent picture.  I would say that imidazole  exists primarily in the cationic state, but there is a significant fraction which is neutral.