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Ch.21 - Transition Elements and Coordination Chemistry
All textbooksMcMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblem 136
Chapter 21, Problem 136

The percent iron in iron ore can be determined by dissolving the ore in acid, then reducing the iron to Fe2+, and finally titrating the Fe2+ with aqueous KMnO4. The reaction products are Fe3+ and Mn2+. (a) Write a balanced net ionic equation for the titration reaction. (b) What is the mass % Fe in the iron ore if titration of the Fe2+ from a 1.265-g sample of ore requires 34.83 mL of 0.051 32 M KMnO4 to reach the equivalence point? (c) Use the E° values in Appendix D to calculate ΔG° (in kilojoules) and the equilibrium constant for the reaction. (d) Does the paramagnetism of the solution increase or decrease as the reaction proceeds? Explain.

Verified step by step guidance
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Step 1: Write the balanced net ionic equation for the titration reaction. The reaction involves Fe^{2+} being oxidized to Fe^{3+} and MnO_4^{-} being reduced to Mn^{2+}. Balance the equation by ensuring the number of electrons lost in oxidation equals the number gained in reduction.
Step 2: Calculate the moles of KMnO_4 used in the titration. Use the formula: moles = concentration (M) × volume (L). Convert the volume from mL to L before calculating.
Step 3: Determine the moles of Fe^{2+} in the sample. Use the stoichiometry of the balanced equation from Step 1 to find the relationship between moles of KMnO_4 and moles of Fe^{2+}.
Step 4: Calculate the mass of iron in the sample. Use the moles of Fe^{2+} and the molar mass of iron (55.85 g/mol) to find the mass of iron.
Step 5: Determine the mass percent of iron in the ore. Use the formula: mass percent = (mass of iron / mass of ore sample) × 100%.

Key Concepts

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

Redox Reactions

Redox reactions involve the transfer of electrons between species, resulting in changes in oxidation states. In this context, iron is reduced from Fe3+ to Fe2+ during the titration with KMnO4, which is itself reduced from MnO4- to Mn2+. Understanding the oxidation states and the electron transfer process is crucial for writing balanced equations and analyzing the reaction.
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Titration and Stoichiometry

Titration is a quantitative analytical method used to determine the concentration of a solute in a solution. In this case, the volume of KMnO4 used in the titration allows for the calculation of the moles of Fe2+ present in the sample. Stoichiometry relates the amounts of reactants and products in a chemical reaction, enabling the determination of mass percent of iron in the ore based on the titration results.
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Gibbs Free Energy and Equilibrium Constant

Gibbs free energy (ΔG°) indicates the spontaneity of a reaction, while the equilibrium constant (K) reflects the ratio of product concentrations to reactant concentrations at equilibrium. The relationship between ΔG° and K is given by the equation ΔG° = -RT ln(K). Calculating these values helps in understanding the thermodynamics of the reaction and predicting its favorability under standard conditions.
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Related Practice
Textbook Question

There are three coordination compounds with the empirical

formula Co1NH3231NO223 in which all the nitrite ions

are bonded through the N atom. Two isomers have the

same molar mass but different nonzero dipole moments.

The third compound is a salt with singly charged ions and a molar mass twice that of the other compounds.

(a) Draw the structures of the isomeric compounds.