   Chapter 4.8, Problem 4.13CYU

Chapter
Section
Textbook Problem

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

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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