Question

In Lab 9, students performed acid-base titrations. Redox reactions can also be used in titrations. An example is the titration of ascorbic acid (H₂C₆H₆O₆) in lemon juice using triiodide (I₃⁻ ). A starch indicator will turn the solution blue-black at the endpoint. The half-reactions involved are shown below.

C6H6O6 + 2 H+ + 2 e−	→	H2C6H6O6	+0.06 V
I3− + 2 e−	→	3 I−	+0.53 V

(a) What is the net redox reaction that occurs? (Use the lowest possible coefficients. Enter electrons in the format "e^-". Omit states-of-matter from your answer.)

(b) What is the stoichiometry of H₂C₆H₆O₆ to I₃⁻?

3:1, 8:3, 2:1, 1:1, 1:2, 3:8, or 1:3

(c) Use the data given below to determine the amount of ascorbic acid in lemon juice. (Note: The recommended daily allowance of ascorbic acid (Vitamin C) is 90 mg.)

Data Table P6: Titration of ascorbic acid in lemon juice with triiodide
concentration of I3−	0.0203 M
volume lemon juice	78.41 mL
mass lemon juice	79.82 g
equivalence volume of I3−	17.51 mL
mmol of I3−	_______ mmol
mmol of H2C6H6O6	_______ mmol
mass of H2C6H6O6	_______ mg

Answer 1

(a) The net redox reaction is : H₂C₆H₆O₆ + I₃^- $\rightarrow$ C₆H₆O₆ + 3 I^- + 2 H⁺

(b) stoichiometry of H₂C₆H₆O₆ to I₃^- is 1 : 1

(c) mmol of I₃^- = (concentration of I₃^-) * (equivalence volume of I₃^-)

mmol of I₃^- = (0.0203 M) * (17.51 mL)

mmol of I₃^- = 0.355 mmol

mmol of H₂C₆H₆O₆ = mmol of I₃^-

mmol of H₂C₆H₆O₆ = 0.355 mmol

mass of H₂C₆H₆O₆ = (mmol of H₂C₆H₆O₆) * (molar mass H₂C₆H₆O₆)

mass of H₂C₆H₆O₆ = (0.355 mmol) * (176.1241 g/mol)

mass of H₂C₆H₆O₆ = 62.6 mg