A solution of Glycine at pH 1.72 was titrated with a NaOH solution. The pH was m

Question

A solution of Glycine at pH 1.72 was titrated with a NaOH solution. The pH was monitored and the results were plotted as shown in the following graph. The key points in the titration are designated in five sections (1) to (V). For each of the statements (a) to (l), Identify the appropriate key point in the titration. Some answer may be used more than once. Glycine is present predominantly as the species "H_3N-CH_2-COOH. The pH is equal to the pKa of the carboxyl group. Glycine has its maximum buffering capacity. The average net charge of glycine is zero, The carboxyl group has been completely titrated (first equivalence point). Glycine is completely titrated (second equivalence point). The predominant species is *H3N-CH2-COO^- The average net charge of glycine is -1. Glycine is present predominantly as a 50:50 mixture of ^+H3N-CH2-COOH and ^+H3N-Ch2-COO^- This is the isolelectric point of glycine. This is the end of the titration. How many equivalents of NaOH are required to complete the titration? What is the predominant ionic form of Glycine at pH 12?

Explanation / Answer

a) Glycine is present predominantly as the species + H3 N-CH2 - COOH ....... No.

b) The pH is equal to pKa of the carbonyl group.............No

c ) The pH is equal to pKa of protonated amino group............................Yes.

d ) Glycine has its maximum buffering capacity............................No

e ) The average net charge of glycine is zero...............................Yes.

f) The carboxyl group has been completely titrated ( first equivalence point )...........No

g) Glycine is completely titrated (second equivalence point )..................No

h) The predominant species is + H3 N -CH2 - COO- ..................Yes.

i) The average net charge of glycine is -1......................No

j) Glycine is present predominantly as a 50-50 mixture of + H3N-CH2-COOH and + H3N -CH2 -COO- ....Yes.

k) This is the isoelectric point of glycine.............at point I

l ) This is the end of titration........................ at point III   

m) How many equivalents of NaOH are required to complete the titration ?...........1.0

n) What is the predominant form of glycine at pH 12 ?...................NH2 CH2 COOH

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