Question

acts like any other polyprotic aliu. Il we will ale s punypruul uuluu determine the pka for each of the potentially acidic sites in that amino acid. Consider the amino acid, glycine shown below in both its neutral form on the left and its fully protonated form on the right: H2N-CH- OH HAN-CH- OH Neutral form of glycine Fully protonated form of glycine (form present at low pH) Here it can act as a diprotic acid In the fully protonated form, there are two acidic sites: Site 1 is the carboxylic acid (RCOOH), pka = 2.34 Site 2 is the protonated amine pka = 9.6 a) What is the Ka for Site 1? 4.5e-3 b) Which of these two sites is more acidic? Site 2 . Site 1 c) Whether these sites are protonated or deprotonated depends on the pH of the solution that they are in. Use the Henderson Hasselbach equation to decide what percentage of site 1 is deprotonated if the surrounding pH= 3.0. 21.87 X % deprotonated

I posted this question earlier and this was the wrong answer

Answer 1

(c) Given : pH = 3.0

pKa = 2.34

According to Henderson - Hasselbalch equation,

pH = pKa + log([conjugate base] / [weak acid])

pH = pKa + log([A-] / [HA])

3.0 = 2.34 + log([A-] / [HA])

log([A-] / [HA]) = 3.0 - 2.34

log([A-] / [HA]) = 0.66

[A-] / [HA] = 100.66

[A-] / [HA] = 4.571

[A-] = 4.571 M

[HA] = 1 M

percentage of deprotonated site = ([A-] / ([A-] + [HA]) * 100

percentage of deprotonated site = (4.571 M / (1 + 4.571)) * 100

percentage of deprotonated site = (0.8205) * 100

percentage of deprotonated site = 82.05 %