In Lab 9, students performed acid-base titrations. Redox reactions can also be u

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.

(a) What is the net redox reaction that occurs? (Use the lowest possible coefficients. Omit states-of-matter from your answer.)

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

3:18:3    2:11:11:23:81:3

(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.)

Explanation / Answer

Ascorbic acid oxidation by I3- to dehydroascorbic acid

(a) net redox reaction

H2C6H6O6 + I3- + H2O ---> C6H6O6 + 3I- + 2H+

(b) stoichiometry of H2C6H6O6 to I3-

= 1 : 1

(c) moles I3- = 0.0205 M x 15.93 ml

                     = 0.33 mmol

moles H2C6H6O6 = 0.33 mmol

mass of H2C6H6O6 = 0.33 mmol x 176.12 g/mol

                                 = 58.12 mg

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