VCE Stuff > VCE Chemistry
In Need of Urgent Help
Collin Li:
--- Quote from: Bear on January 06, 2008, 04:56:57 pm ---Can you please show me how do you know whether the amino acid is acting as a acid or base? Because all the information we got was the formula for Glycine. Can you show me the working when we are finding the overall charge when the amino acid is acting as a base? Say, when pH = 4.3? Thanks Coblin.
--- End quote ---
Sorry, I wouldn't have a clue. You have two pKa values, so it might be competing. I'd roughly guess that you take the midpoint of the two pKa's () and if the pH is below 5.95, you'd work with pKa = 2.3, and if the pH is above 5.95, you'd work with pKa = 9.6.
This is a total guess, by the way. I never really did this in great detail.
Collin Li:
What we could try is analysing both situations, and getting a three-way ratio.
Say the pH is 4.3:
Looking at the NH2 (amine) group
Now, we could look at the COOH (carboxy) group:
Now, with both of these ratios, we can combine them to get:
(The two species in the middle are the same thing, just written back to front.)
So, we have a ratio between the charged species. This means that there will be an overall charge of to 3 significant figures. The acidic form strongly overpowers the basic form.
Bear:
Actually...i think we find the charge of the carboxyl group, and the amino group, and add it together to find the net charge on glycine..?
Collin Li:
Possibly. I'm not really sure how this works, because I've never done these calculations for a substance with two different pKa values.
Bear:
Bloody hell. Who could have thought unit 1/2 chemistry was so hard
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