Question

From your plot, estimate the pKa values for the a-COOH and a-NH3+ groups of Gly. Use arrows to indicate these pKa values on the curve:

pH vs. Accumulated Equivalents of NaOH 12 10 6 2 2 4 6 10 12 Accumulated Equivalents of NaOH (mL) O Hd

Answer 1

Solution:

Titration of glycine against NaOH look likes the titration of weak diprotic acid. In the given titration curve of glycine agaonst NaOH, two pKa's corresponding to COOH and NH3 groups are:

Pka1 = 2.30 ( COOH)

Pka2 = 9.70 (NH3+)

Reason:

The first equivalence point at 6.1 mL of NaOH at pH=6

Thus,  half equivalence point (at 3.05 mL NaOH) , pH = pKa

Hence, pH corresponding to 3.05 mL NaOH = 2.30

Therefore, pKa (COOH) = 2.30

And , the next half equivalence point obtained at 9.70

Thus, pKa.= 9.70 (NH3+).