4. A 100 mL solution of 0.1 M glycine at pH 1.72 was titrated with 2 M NaOH solu

Question

4. A 100 mL solution of 0.1 M glycine at pH 1.72 was titrated with 2 M 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 I to V. For each of the statements (a) to (d), identify the appropriate key point in the titration and justify your choice 12 (a) Glycine is present predominantly as the species "H3NCH2COOH (b) The average net charge of glycine is +1/2. (c) The worst pH region for the buffer power. (d) The pH is equal to the pKa of the protonated amino group 10 9.60__--IV pH 6 5,97- 2.34 0.5 2.0 OH (equivalents)

Explanation / Answer

A) The correct option would be (I) since both of the groups in glycine, NH2 and COOH are protonated and this is the only pH where it can occur. The COOH group would become COO- predominantly above (II).

B) This charge would be possible at region II because at this ph, Amino group is fully protonated giving +1 charge while the carboxyl group is half protonated giving -1/2 charge leading to a sum total of +1/2 charge.

C) Region III is the worst buffer region as the pH changes very quickly with the addition of OH- and a buffer should be able to maintain the pH to some extent whenever OH- or H+ is added to the solution which would otherwise change the pH.

D) Since the Pka of the amino group is 9.6, the region would be IV.

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