Given pH Value along with PKA Values?

[Alpha Centauri]

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Hey guys. I have come across questions, in destroyer for instance, where a pH / pKA is given and we need to determine which ones would be deprotonated or not. I have looked at the solutions but the concept is still kind of fuzzy. Can anyone explain?

 

[rdub]

The best way to think about it for the DAT is this: If the pH is less than the pKa, it will be protonated. If the pH is higher than the pKa, it will be deprotonated.

For example, the pKa of H30+ is ~2. If the H3O+ is put into a solution with a pH lower than 2, then it stays as H3O+. If it is put into a solution with a pH higher than 2, it is deprotonated and becomes H2O. Now, water has a pKa of ~16. So once you get to a pH higher than 16 it becomes OH-.

Make sense?

 

[Alpha Centauri]

The best way to think about it for the DAT is this: If the pH is less than the pKa, it will be protonated. If the pH is higher than the pKa, it will be deprotonated.

For example, the pKa of H30+ is ~2. If the H3O+ is put into a solution with a pH lower than 2, then it stays as H3O+. If it is put into a solution with a pH higher than 2, it is deprotonated and becomes H2O. Now, water has a pKa of ~16. So once you get to a pH higher than 16 it becomes OH-.

Make sense?

Yes put very simply thank you! Just need to find some questions to solidify the concept now. If water was in a ph of 15 would that mean it would become protonated?

 

[rdub]

Yes put very simply thank you! Just need to find some questions to solidify the concept now. If water was in a ph of 15 would that mean it would become protonated?

At pH 15 it would stay as H2O which is considered the protonated form. Look at it this way:

It is H3O+ when the pH is less than or equal to 2
It is H2O when the pH is between 2 and 16
It is OH- when the pH is 16 or above.

 

[DAT Destroyer]

Hey guys. I have come across questions, in destroyer for instance, where a pH / pKA is given and we need to determine which ones would be deprotonated or not. I have looked at the solutions but the concept is still kind of fuzzy. Can anyone explain?

Let us do an example,,,,you are sure to be delighted. Consider Acetic Acid....the pKa is 4.7. If you are below this number on the pH scale,,,,say in the stomach where the pH is 1.5,,,,,the molecule would be mainly PROTONATED since there are so many hydrogen ions around. At a pH of say 11....we are well beyond the pKa value.....hence the large majority of molecules will be DEPROTONATED, and it will exist as the carboxylate salt Ch3COO-,,,,,and if you are EXACTLY at a pH of 4.7.....the same value as the pKa.....this means you are at the Half Equivalence Point......you have 50% of the molecules protonated and 50% deprotonated,,,,,hence you are at the ideal buffer zone. Different functional groups have different pKa values. Carboxy Acids have values around 5..... For amino acids.....the acid group is around 2.3 and the NH3+ group is around 9.6. ....This is a very important concept !!!! You will need this for BioChemistry !

Hope this helps

Dr. Romano