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

Question

In Lab. 9, students performed acid-base titrations. Redox reactions can be used in titrations. An example is the titration. An example is the titration of ascorbic acid (H_2C_6H_6O_6) in lemon juice using trioxide (I_3^-). A starch indicator will turn the solution blue-black at the endpoint. The half-reactions involved are shown below. C_6H_6O_6 + 2 H^+ + 2 e^- rightarrow H_2C_6H_6O_6 + 0.06 V I_3^- + 2 e^- rightarrow 2 I^- +0.53 V (a) What is the net redox reaction that occurs?(Use the lowest possible coefficients. Omit states-of matter from your answer) (b) What the stoichiometry of H_2C_6H_6O_6 to I_3^-? 3:1 8:3 2:1 1:1 1:2 3:8 1:3 Use the data given below to determine the amount of ascorbic acid in lemon juice.

Explanation / Answer

First, identify redox reaction

reduction --> highest potential

I3- + 2e- --> 3I- will happen

so

oxidation is

H2C6H6O6 --> C6H6O6 + 2H+ + 2e-

Ovarall reaction

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

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

b)

stoichiometry ratio

1 mol of acid per 3 mol of Iodine

so

1:3

c)

mmol = M V= 0.0195*16.12 = 0.3143 mmol of I3-

mmol of acid --> 1:3 ratio so -->  0.3143 /3 = 0.1047 mmol of acid

mass of acid = mmol*Mw = (0.1047)*176.12 = 18.439 mg of acid

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