The graph below represents the titration of an amino acid with NaOH solution. Vi

Question

The graph below represents the titration of an amino acid with NaOH solution. View the titration and answer the questions below (parts a through e). At what pH are about 1/2 of the amino groups ionized? Where does the amino acid have a net charge of -1? above pH 9. 69 at pH = 9.69 at pH = 6.01 at pH = 2.34 below pH 2.34 At what point has enough base been added to react with 1/2 of the COOH groups? Equivalent OH^- PH = At which of the following pH values does the amino acid have the best buffering capacity? 3.00 9.69 1.53 10.5 9.00 6.01 What is the pI (isoelectric point)?

Explanation / Answer

a)

Half of the amine groups are ionized at pH = pKa of amine

pKa of amine = 9.69 so half of the amino group is ionized at pH = 9.69

b)

Amino acid have net charge -1 when dissociable protons are completely titrated with base

So it must be above pH =9.69.

c)

Half of the COOH is titrated at pH = pKa of COOH

pH = 2.34

and equivalents of OH- = 0.5 equivalent OH-

d)

Best buffering capacities for this amino acid have two points

1) 2.34 and 2) 9.69

We have only one option so answer is 9.69

e)

pI = mid point on graph

or we can say avarage of two pKa values

So answer is pI= 6.01

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