A solution of alanine hydrochloride, [H3NCH(CH3)COOH]+Cl-, is titrated with a solution of sodium hyd — Acid-Base Equilibrium / Titration Chemistry Question
Problem Context
A solution of alanine hydrochloride, [H3NCH(CH3)COOH]+Cl-, is titrated with a solution of sodium hydroxide to produce a curve similar to the one shown.
Give the formula(s) for the major species present at the points on the titration curve.
i. 1
Model Answer
[H3NCH(CH3)COOH]+ and +H3NCH(CH3)COO–
ii. 2
Model Answer
+H3NCH(CH3)COO–
iii. 3
Model Answer
+H3NCH(CH3)COO– and H2NCH(CH3)COO–
iv. 4
Model Answer
H2NCH(CH3)COO–
If K1 and K2 of alanine hydrochloride are 4.6×10–3 and 2.0×10–10 respectively,
i. write equations to represent the reactions responsible for K1 and K2.
Model Answer
K1 [H3NCH(CH3)COOH]+ → H+ + +H3NCH(CH3)COO–
K2 +H3NCH(CH3)COO– → H+ + H2NCH(CH3)COO–
ii. determine the pH at points 1, 2 and 3.
Model Answer
[H+] at point 1 = K1 = 4.6×10-3 so pH = 2.34
[H+] at point 3 = K2 = 2×10-10 so pH = 9.07
Describe quantitatively how you could prepare a buffer solution with a pH = 10.0. Solutions of 0.10 M alanine hydrochloride and 0.10 M NaOH are available.
Model Answer
Take a specified volume of 0.10 M alanine hydrochloride(such as 300 mL) in which the predominant species is [H3NCH(CH3)COOH]+. Add 0.10 M NaOH so there is 5/3 as much as there is 0.10 M alanine (such as 500 mL). This will give 800 mL of solution with a ratio of .