Limonite is an ore of iron that contains 2 Fe2 O3 ·3 H2 O. A 0.5166-g sample of limonite is diss

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Limonite is an ore of iron that contains 2 Fe2 O3 ·3 H2 O. A...

Limonite is an ore of iron that contains $2 \mathrm{Fe}_{2} \mathrm{O}_{3} \cdot 3 \mathrm{H}_{2} \mathrm{O}$. A $0.5166-\mathrm{g}$ sample of limonite is dissolved in acid and treated so that all the iron is converted to ferrous ions, $\mathrm{Fe}^{2+}$. This sample requires $42.96 \mathrm{~mL}$ of $0.02130 \mathrm{M}$ sodium dichromate solution, $\mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$, for titration. $\mathrm{Fe}^{2+}$ is oxidized to $\mathrm{Fe}^{3+}$, and $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}$ is reduced to $\mathrm{Cr}^{3+}$. What is the percent iron in the limonite? If your answer had been over $100 \%$ limonite, what conclusion could you make, presuming that the analytical data are correct?

Key Concepts

Redox Titration

Redox titration is an analytical technique that involves oxidation?reduction reactions to determine the concentration of an analyte. In a redox titration, a titrant with a known concentration is added to a solution containing the target substance, leading to a redox reaction. By measuring the volume of titrant required to reach the endpoint, the amount of the analyte present can be calculated using the stoichiometry of the reaction.

Stoichiometry of Redox Reactions

Stoichiometry in redox reactions involves using the balanced chemical equations to relate the moles of reactants and products. This is essential in analytical procedures since it allows for the conversion between the amount of titrant used and the amount of analyte present. The stoichiometric ratios provide the foundation for determining concentration and composition in a sample.

Percent Composition Analysis

Percent composition analysis is a method used to determine the proportion by mass of each element within a compound or mixture. In the context of mineral samples, it is used to calculate the percentage of a particular element by comparing the mass of that element obtained from the analysis to the total mass of the sample, often after carrying out titration or other quantitative chemical procedures.

Mole Calculations in Analytical Chemistry

Mole calculations are fundamental in analytical chemistry, where the concept of the mole is used to convert between mass and the number of particles or molecules. By using molarity (concentration) and volume measurements, analysts can translate these quantities into moles, which are then employed in stoichiometric calculations to deduce the quantities of substances present in a chemical sample.

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