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Bear:
Doing some summer study ...
Using Henderson-Hassalbach Equation
(Remember pH = pKa + ([Base] / [Acid] )

Glycine is NH2 - Ch2 - COOH
Amino Group has pKa = 9.6
at pH = 9.6
Whats the charge on NH3+ ?

Their working was
9.6-9.6 = 0 = log10 ([NH2]/[Nh3+]
1 = [Nh2] / [Nh3+] = [0.5 NH2] / [0.5 NH3+ ]
Charge = +0.5
Which i totally don't get! Any ideas?

Collin Li:
I was not aware that anything like this was on the course, but anywho, this is what I believe is going on:

You have an amino acid with two forms, the protonated HOOC-CH2-NH3+ acidic form, or the HOOC-CH2-NH2 conjugate base.

Using the formula given, subbing in pH = pKa = 9.6, you get:







This means there is a 1:1 ratio of the acid and the base. This means that there is 50% of the +1 charged species, and 50% of the neutral species. This means the overall 'average' charge on the amino acid is +0.5.

EDIT: By the way, this will be moved to the Chemistry board.

Bear:
Thanks COBLIN

Collin Li:
No problem :)

Sorry, can I just ask you, was that a VCE question? It doesn't look like it to me... are you a university student?

Bear:
Can you show me what the charges on Nh3+ will be, say if the pH was 6.6 and 12.6 ?

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