   Chapter 4, Problem 107GQ

Chapter
Section
Textbook Problem

Sodium thiosulfate, Na2S2O3, is used as a “fixer” in black-and-white photography. Suppose you have a bottle of sodium thiosulfate and want to determine its purity. The thiosulfate ion can be oxidized with I2 according to the balanced, net ionic equationI2(aq) + 2 S2O32−(aq) → 2 I−(aq) + S4O62− (aq)If you use 40.21 mL of 0.246 M I2 in a titration, what is the weight percent of Na2S2O3 in a 3.232-g sample of impure material?

Interpretation Introduction

Interpretation:

The weight percentage of Na2S2O3 in the sample of given material has to be determined.

Concept introduction:

• 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.
• 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).
• 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
• Limiting reagent: Limiting reagent is a reactant, which consumes completely in the chemical reaction. The quantity of the product depends on this limiting reagent, it can be determined with the help of balanced chemical equation for the reaction
• Weight percent of elements of a compound is the ratio of weight of element to the weight of whole compound and multiplied with hundred.
Explanation

Balanced chemical equation for the given reaction is,

I2(aq)+2S2O23(aq)2I(aq)+S4O62(aq)

Amount of I2 can be calculated from its volume and concentration as follows,

AmountofI2 =CI2×VI2 =0.246molL×0.04021L =0.00989molI2

Using the stoichiometric factor the amount of S2O32- can be calculated as follows,

That is,

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