Correlating pka and pH with ionic forms (exercises)

In summary, at pH=6, the form of benzotriazole cannot be determined based on the given information. The pKa value of 12 is incorrect and the molecule behaves differently from ordinary amines. Depending on the interpretation, the correct answer could be cationic or neutral, but there is no clear answer.
  • #1
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I have the following molecule: benzotriazole. It has pka=12. At pH=6 which form prevails?

a) neutral
b) anionic
c) cationic
d) none of the above.

I thought the correct answer was c) cationic: pH<pka, benzotriazole has basic functional groups (amines).
 

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  • #2
I don't know where your figure of 12 comes from. Wikipedia quotes a value of 8.2. Moreover, this is the pKa of the neutral molecule, not the conjugate acid. Benzotriazole behaves very differently from ordinary amines:
"Moreover, the proton does not tightly bind to any of the nitrogen atoms, but rather migrates rapidly between positions 1 and 3. Therefore, the BTA can lose a proton to act as a weak acid (pKa = 8.2)[3][4] or accept a proton using the lone pair electrons located on its nitrogen atoms as a very weak Bronsted base (pKa < 0)." (https://en.wikipedia.org/wiki/Benzotriazole)
People sometimes talk about the "pKa of an amine" when they mean the pKa of its conjugate acid, but this is sloppy, and in this case actually incorrect.
 
  • #3
mjc123 said:
I don't know where your figure of 12 comes from. Wikipedia quotes a value of 8.2. Moreover, this is the pKa of the neutral molecule, not the conjugate acid. Benzotriazole behaves very differently from ordinary amines:
"Moreover, the proton does not tightly bind to any of the nitrogen atoms, but rather migrates rapidly between positions 1 and 3. Therefore, the BTA can lose a proton to act as a weak acid (pKa = 8.2)[3][4] or accept a proton using the lone pair electrons located on its nitrogen atoms as a very weak Bronsted base (pKa < 0)." (https://en.wikipedia.org/wiki/Benzotriazole)
People sometimes talk about the "pKa of an amine" when they mean the pKa of its conjugate acid, but this is sloppy, and in this case actually incorrect.

No.12 was given by our teacher ( this exercise was in the previous exam)...it’s more like hypothetically speaking. They just want to test if we know which form that molecule is in at pH x.

So...I guess in this case the correct answer is d).
 
  • #4
In the circumstances, there is no correct answer. I don't know what your teacher was thinking. If it was a normal amine with pKa of conjugate acid 12, it would be cationic. If it was the pKa of the neutral amine (but with the wrong value), the answer would be neutral.
 

Related to Correlating pka and pH with ionic forms (exercises)

1. How do I determine the pKa value of a compound?

The pKa value of a compound can be determined experimentally by measuring the pH at which the compound is 50% protonated and 50% deprotonated. This is known as the half-equivalence point. Alternatively, the pKa value can be calculated using the Henderson-Hasselbalch equation, which relates the pH, pKa, and ratio of protonated to deprotonated forms of a compound.

2. What is the relationship between pKa and pH?

pKa and pH are related by the Henderson-Hasselbalch equation, which states that the pH of a solution is equal to the pKa plus the logarithm of the ratio of protonated to deprotonated forms of a compound. This means that as the pKa value increases, the pH of a solution also increases.

3. How does the ionic form of a compound affect its pKa and pH?

The ionic form of a compound can greatly affect its pKa and pH. For example, a compound with a positive charge (cation) will have a lower pKa and a lower pH compared to its neutral form. Similarly, a compound with a negative charge (anion) will have a higher pKa and a higher pH compared to its neutral form.

4. Can pKa and pH be used to predict the behavior of a compound in a solution?

Yes, pKa and pH can be used to predict the behavior of a compound in a solution. The pKa value can give an indication of the strength of an acid or base, while the pH of a solution can determine the degree of ionization of a compound. This information can be useful in understanding how a compound will interact with other molecules in a solution.

5. How can I use exercises to practice correlating pKa and pH with ionic forms?

There are various exercises and practice problems available online that can help you improve your understanding of correlating pKa and pH with ionic forms. These exercises typically involve calculating the pH of a solution given the pKa value of a compound, or vice versa. Additionally, you can create your own exercises by selecting different compounds and their corresponding pKa and pH values, and then calculating the ratio of protonated to deprotonated forms using the Henderson-Hasselbalch equation.

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