   # Vitamin C, ascorbic acid (C 6 H g O 6 )(molar mass 176.1 g/mol), is a reducing agent. One way to determine the ascorbic acid content of a sample is to mix the acid with an excess of iodine, C 6 H g O 6 (aq) + I 2 (aq) + H 2 O(l) → C 6 H g O 6 (aq) + 2 H 3 O + (aq) + 2 I – (aq) and then titrate the iodine that did not react with the ascorbic acid with sodium thiosulfate. The balanced, net ionic equation for the reaction occurring in this titration is I 2 (aq) + 2 S 2 O 3 2– (aq)→2 I – (aq) + S 4 O 6 2– (aq) Suppose 50.00 mL of 0.0520 M I 2 was added to the sample containing ascorbic acid. After the ascorbic acid/I 2 , reaction was complete, the I 2 not used in this reaction required 20.30 mL of 0.196 M Na 2 S 2 O 3 for titration to the equivalence point. Calculate the mass of ascorbic acid in the unknown sample. ### Chemistry & Chemical Reactivity

9th Edition
John C. Kotz + 3 others
Publisher: Cengage Learning
ISBN: 9781133949640

#### Solutions

Chapter
Section ### Chemistry & Chemical Reactivity

9th Edition
John C. Kotz + 3 others
Publisher: Cengage Learning
ISBN: 9781133949640
Chapter 4.7, Problem 5CYU
Textbook Problem
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## Vitamin C, ascorbic acid (C6HgO6)(molar mass 176.1 g/mol), is a reducing agent. One way to determine the ascorbic acid content of a sample is to mix the acid with an excess of iodine,C6HgO6(aq) + I2(aq) + H2O(l) → C6HgO6(aq) + 2 H3O+(aq) + 2 I–(aq)and then titrate the iodine that did not react with the ascorbic acid with sodium thiosulfate. The balanced, net ionic equation for the reaction occurring in this titration isI2(aq) + 2 S2O32–(aq)→2 I–(aq) + S4O62–(aq)Suppose 50.00 mL of 0.0520 M I2 was added to the sample containing ascorbic acid. After the ascorbic acid/I2, reaction was complete, the I2 not used in this reaction required 20.30 mL of 0.196 M Na2S2O3 for titration to the equivalence point. Calculate the mass of ascorbic acid in the unknown sample.

Interpretation Introduction

Interpretation:

The mass of ascorbic acid in the unknown sample should be determined.

Concept introduction:

• The molar mass of an element or compound is the mass in grams of 1 mole of that substance, and it is expressed in the unit of grams per mol (g/mol).
• For chemical reaction balanced chemical reaction equation written in accordance with the Law of conservation of mass.
• Law of conservation of mass states that for a reaction total mass of the reactant and product must be equal.
• Stoichiometric factor is a relationship between reactant and product which is obtained from the balanced chemical equation for a particular reaction.
• Concentrationofsubstance=Amountof substancevolumeofthesubstance
• Amountof substance=Concentrationofsubstance×volumeofthesubstance
• Titration is a method to determine the concentration of a substance in solution by adding a solution of known volume and concentration until the reaction is completed.
• Mass percent of elements of a compound is the ratio of mass of element to the mass of whole compound and multiplied with hundred.

### Explanation of Solution

The balanced net ionic equation for the reaction occurring in the titration is,

I2(aq)+2S2O3(aq)22I(aq)+S4O6(aq)2

Amount of Na2S2O3 can be determined as follows,

Amount ofNa2S2O3 =CNa2S2O3×VNa2S2O3 = 0.196molNa2S2O31L×0.02030L = 0.003979molNa2S2O3

From the amount of Na2S2O3 number of moles of I2 can be calculated.  Stoichiometric ration between Na2S2O3 and I2 is 2:1

0.003979molNa2S2O3×1molI22molNa2S2O3=0.001989molI2

0.001989molI2 is in excess and was not used in the reaction with ascorbic acid.

Thus,

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