L-leucine hydrochloride, [(CH3)2CHCH2CH(NH3)COOH]+ Cl– , is used as the dietary supplement for the e — Acid-Base Equilibrium / Titration Chemistry Question
Problem Context
L-leucine hydrochloride, [(CH3)2CHCH2CH(NH3)COOH]+ Cl– , is used as the dietary supplement for the essential amino acid leucine. It is a diprotic acid with pKa1 = 2.36 and pKa2 = 9.60. To 50.0 mL of a 0.100 M solution of leucine hydrochloride is gradually added 0.500 M NaOH solution.
What is the initial pH of the 0.100 M leucine hydrochloride solution?
Model Answer
LeuH2+ (aq) ⇌ LeuH (aq) + H+ (aq) Ka = 10–2.36 = 4.4 × 10-3
If x = [H+] = [LeuH], then [LeuH2+] = 0.100 – x.
x = 0.019 M = [H+]
pH = –log10[H+] = 1.72
Since ~20% of the leucine hydrochloride is ionized, neglecting its ionization (i.e., assuming [LeuH2+]final = [LeuH2+]initial) is not a great approximation. If this approximation is made, then one calculates [H+] = 0.021 M, pH = 1.68.
What volume of 0.500 M NaOH must be added to achieve a pH of 3.50?
Model Answer
3.50 = 2.36 + log10 [LeuH] / [LeuH2+]
(mol LeuH)/(mol LeuH2+) = 13.8
Since (mol LeuH) + (mol LeuH2+) = 5.00 × 10-3 mol, then we must have 4.66 × 10-3 mol LeuH, which would have required 4.66 × 10-3 mol NaOH to form.
Volume added NaOH = (4.66 × 10-3 mol)/(0.500 mol/L) = 9.32 mL.
After being deprotonated once, leucine hydrochloride forms a species with an overall charge of zero. Draw the Lewis structure of this neutral species.
What is the maximum concentration of this species in the titration?
Model Answer
The concentration of this species is maximized at the first equivalence point, where 10.0 mL NaOH have been added. [LeuH] = 0.00500 mol/0.0600 L = 0.0833 M.
For each of the indicators listed below, state whether its color will change abruptly (over the course of addition of < 0.2 mL NaOH) during the titration of leucine hydrochloride. If it will, specify the volume of NaOH at which the color change will take place. If it will not, explain why not.
Methyl red (color change from pH = 4.4 to 6.2)
Phenolphthalein (color change from pH = 8.3 to 10.0)
Model Answer
Methyl red: Will change pH abruptly at the first equivalence point (where the pH will jump abruptly from about 4.4 to about 7.6), at 10.00 mL added NaOH.
Phenolphthalein: The buffer range of LeuH/Leu– is from about pH = 8.6 to 10.6. This is where the pH will change very slowly as NaOH is added, so the color of phenolphthalein will change very slowly as well.