D. At which of the following PH values does the amino acid have the best bufferi
ID: 145497 • Letter: D
Question
D. At which of the following PH values does the amino acid have the best buffering capacity?
E. What is the pi (isoelectric point)
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) (a) At what pH is the average net charge -12? Number 6-969 2.0d (b) Where does the amino acid have a net charge of -1? O above pH 9.69 O at pH 9.69 O at pH = 6.01 O at pH 2.34 O below pH 2.34 p-234 20 OH (equivalents) (c) At what point has enough base been added to react with 2 of the NH3' groups? continuExplanation / Answer
a) at pH 9.69 the average net charge will be -1/2. At that pH half are protonated and half deprotonated so the equation will be -1+(1-1/2) = -1+1/2= -1/2
b) above pH 9.69
When pH is raised above its pI, the nitrogen in -NH3+ group starts to lose its proton and thus its positive charge. So it becomes negatively charged. Half of them are protonated and half deprotonated once they reach at pH 9.69 and well above pH 9.69 they lose all protons and become negative charged -1.
c) at pH 2.3 enough base has been added to react with the 1/2 of the amino groups. At pH 2.3 half of them are protonated and half deprotonated. So, 1+(-1+1/2) = 1/2
D. At pH 6.01, the amino acid has the best buffering capacity. The zwitterionic structure of amino acis has got zero charge. At pI value of 6.01 it has no charge. This makes the carboxylate group to act as a base and protonated amino group to become an acid. Hence it can react with both acid and base and makes best buffers.
E) pI or isoelectric point: The isoelectric point is the pH at which a particular molecule carries no net electric charge or it is electrically neutral. In the above example, the amino acid has pI=6.01.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.