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Author Topic: pH for a glycine solution  (Read 9204 times)

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THC

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pH for a glycine solution
« on: January 15, 2008, 04:45:16 AM »

The amino acid glycine is dissolved in water: c(glycin) = 0.200 M. Calculate pH for the solution.

Glycine on wiki: http://en.wikipedia.org/wiki/Glycine
I looked up pKa/pKb for the compound:
pKa = 9,87
pKb = 11,65
But how do I use this? I know that glycine (aq) is protonated: +NH3-CH2-COO-. How do I calculate pH? Should I use pH = ½(pKs + pKs(amph)), since glycine is a ampholyte?
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0000000

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Re: pH for a glycine solution
« Reply #1 on: February 02, 2008, 08:12:13 AM »

I see that you are not *deleted*, may be is late but I will help you

first of all write the dissociation of the substance, and you will see that it's dissolved in to two kind of ions, bought of them will react with the solevend formating an acid and a base.
We can make an approximation and we will say that the substance is totally dissolved (because you don't have K for this equilibrium), and  in the solution there will be two equilibrium one with the acid and its conjugate base and the other one the base and it's conjugate acid. The constants for them are pKb and pKa, or precisely 10^-pKa or 10^-pKb.
In one of the equilibrium there are OH ions, you will change them with H ions using the fact that the concentrations of the H and the conc. of the OH ions is connected with this relation pH+pOH=14 (at 25C). And after that you will calculate the concentration of H ions and to find the pH of the solution. There is another way, but for beggin-er is good to realize how is going the process
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