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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: phineasiquit on October 13, 2010, 01:41:06 PM

Title: zwitterions, strong acids, and pH
Post by: phineasiquit on October 13, 2010, 01:41:06 PM

Hi,

So we have glutamic acid (HO₂C(CH₂)₂CHCOOHNH₂) in solution and we add an equal amt of moles of HCl and we want the final pH. Glutamic acid has three pKas, 2.2, 4.2, and 9.7 and a pHi of 3.2. Here's my reasoning but it's not producing the right answer. What is wrong in my reasoning?:

When we add the glutamic acid to the water, the majority species will charge -1, because at water's pH two of the three H's will dissociate. The only protonated function is the NH₃⁺. So then, when we add equal amount of HCl, its freed protons will attach to the 2.2 pKa functions of the glutamic acid and we'll end up with the zwitterion in H₂O. That gives us a final pH of pHi = 3.2.

But the correct answer we're given is that we end up with the same number of moles of a fully protonated (thus charge +1) glutamic acid species and hence the pH is that of a weak acid, .5pKa - .5log[species] etc. What is going on here?

Thanks!