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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: sailpatrick on September 28, 2009, 07:57:26 AM

Title: Amino acid pK and pH question
Post by: sailpatrick on September 28, 2009, 07:57:26 AM
Hey guys!

I have a question regarding this amino acid :                                                            O
                                                                                                                        ||       (pK1=2.2)
  H2N --- C---NH---CH2---CH2---CH2---CH--C---OH
                            |                                                                                      |
                            NH2                                                                  NH3 (pK2 =9)
 pK3=12.5)   

I need to know why these 2 answers would be incorrect ( with explanation) regarding the above aa
1) the net charge is -1 within the pH range 2.2-9.0

edit: the forum doesn't seem to keep my drawing in place with the substituents so the NH2 belongs under the first C from the left  and the NH3 belongs under the first CH from the right and the double bonded O of course belongs above the first C from the right . Sorry about the confusion
2) the form of amino acid shown predominates for pH>2.2

Thanks a bunch!
Title: Re: Amino acid pK and pH question
Post by: Yggdrasil on September 28, 2009, 12:19:44 PM
First consider what the protonated and deprotonated forms of the amine and carboxylic acid groups look like and what charge they have.
Title: Re: Amino acid pK and pH question
Post by: KritikalMass on October 09, 2009, 05:53:01 PM
Do you know what the pK represents? What happens when pH = pK?