Ch.4 - Reactions in Aqueous Solution

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.4 - Reactions in Aqueous Solution

Problem 138

Chapter 4, Problem 138

A sample weighing 14.98 g and containing a small amount of copper was treated to give a solution containing aque-ous Cu2+ ions. Sodium iodide was then added to yield solid copper(I) iodide plus I3 with thiosulfate, S2O3 - ion, and the I3 - was titrated 2-. The titration required 10.49 mL of 0.100 M Na2S2O3 for complete reaction. What is the mass percent copper in the sample? The balanced equations are

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Identify the relevant chemical reactions involved in the process. The first reaction is the formation of copper(I) iodide and triiodide ions: \[ 2 \text{Cu}^{2+} + 4 \text{I}^- \rightarrow 2 \text{CuI} + \text{I}_3^- \]. The second reaction is the titration of triiodide ions with thiosulfate: \[ \text{I}_3^- + 2 \text{S}_2\text{O}_3^{2-} \rightarrow 3 \text{I}^- + \text{S}_4\text{O}_6^{2-} \].

Calculate the moles of thiosulfate used in the titration. Use the formula \( \text{moles} = \text{Molarity} \times \text{Volume} \). Convert the volume from mL to L by dividing by 1000.

Determine the moles of \( \text{I}_3^- \) that reacted. From the balanced equation, 1 mole of \( \text{I}_3^- \) reacts with 2 moles of \( \text{S}_2\text{O}_3^{2-} \). Use this stoichiometric relationship to find the moles of \( \text{I}_3^- \).

Relate the moles of \( \text{I}_3^- \) to the moles of \( \text{Cu}^{2+} \). From the first balanced equation, 1 mole of \( \text{I}_3^- \) is produced from 2 moles of \( \text{Cu}^{2+} \). Calculate the moles of \( \text{Cu}^{2+} \).

Calculate the mass percent of copper in the sample. First, find the mass of copper using its molar mass and the moles calculated. Then, use the formula \( \text{mass percent} = \left( \frac{\text{mass of Cu}}{\text{total mass of sample}} \right) \times 100 \) to find the mass percent.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions. It involves using balanced chemical equations to determine the relationships between the amounts of substances consumed and produced. In this question, stoichiometry is essential for relating the moles of Cu2+ ions to the moles of thiosulfate used in the titration, allowing for the determination of the mass of copper in the original sample.

Titration

Titration is a quantitative analytical technique used to determine the concentration of a solute in a solution. In this case, sodium thiosulfate (Na2S2O3) is used to titrate the iodine (I3-) produced from the reaction of copper(I) iodide with iodide ions. Understanding the titration process, including the stoichiometry of the reaction and the endpoint determination, is crucial for calculating the amount of copper present in the sample.

Mass Percent Composition

Mass percent composition is a way to express the concentration of an element in a compound or mixture as a percentage of the total mass. It is calculated by dividing the mass of the element by the total mass of the sample and multiplying by 100. In this problem, after determining the mass of copper from the titration results, calculating the mass percent of copper in the original 14.98 g sample is necessary to answer the question.

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