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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Kork on March 21, 2012, 12:18:50 PM

Title: pKa-values
Post by: Kork on March 21, 2012, 12:18:50 PM
How can a compound have 2 pKa-values?

Im working with glycerole-3-phosphate which have two pKa-values 2,15 and 7,2. How can it have two Pka-values. And how do I figure which kind of the compound there will be if it's put in a solution with pKa=4?
Title: Re: pKa-values
Post by: sjb on March 21, 2012, 12:23:57 PM
Quote from: Kork on March 21, 2012, 12:18:50 PM
How can a compound have 2 pKa-values?

Im working with glycerole-3-phosphate which have two pKa-values 2,15 and 7,2. How can it have two Pka-values. And how do I figure which kind of the compound there will be if it's put in a solution with pKa=4?

What is the definition of pKa?
Title: Re: pKa-values
Post by: Borek on March 21, 2012, 12:26:15 PM
If it is diprotic it has two pKa's, if it is triprotic it has three, if it is tetraprotic it has four and so on.

I guess you mean pH=4, not pKa=4, as pKa is not a property of a solution, but of the substance.

Knowing pH and Ka (pKa) you can calculate relative concentrations of acid and conjugate base, it requires just a simple rearranging of a dissociation constant.
Title: Re: pKa-values
Post by: Kork on March 21, 2012, 01:28:26 PM
yes i mean pH=4 will it be protonated´? why?
Title: Re: pKa-values
Post by: Borek on March 21, 2012, 01:57:29 PM
Quote from: Kork on March 21, 2012, 01:28:26 PM
yes i mean pH=4 will it be protonated´? why?

What do you mean by "it"? Do you understand that "protonated" in the case of diprotic acid is ambiguous?
Title: Re: pKa-values
Post by: Kork on March 21, 2012, 02:37:37 PM
how do I figure which kind of the compound there will be if it's put in a solution with pH=4?

this is what my teacher asked!
Title: Re: pKa-values
Post by: Borek on March 21, 2012, 02:42:36 PM
What species are present in the solution of diprotic acid H2A?