A 0.500 g piece of brass (copper and zinc) is dissolved in acid. An excess of KI is added, which reacts only with the copper: 2 Cu²⁺ + 4 I⁻ → 2 CuI + I₂. The iodine set free needs 47.2 mL of 0.100 mol L⁻¹ sodium thiosulfate: I₂ + 2 S₂O₃²⁻ → 2 I⁻ + S₄O₆²⁻. What is the percentage of copper in the brass by mass, to the nearest integer? Atomic masses in g mol⁻¹: Cu 63.5.

ChemistryHardHigh School (11–12)Atoms and Molecules › Stoichiometry and Concentration Olympiad

A $0.500\ \text{g}$ piece of brass (copper and zinc) is dissolved in acid. An excess of KI is added, which reacts only with the copper: $2\,\text{Cu}^{2+} + 4\,\text{I}^- \rightarrow 2\,\text{CuI} + \text{I}_2$ . The iodine set free needs $47.2\ \text{mL}$ of $0.100\ \text{mol L}^{-1}$ sodium thiosulfate: $\text{I}_2 + 2\,\text{S}_2\text{O}_3^{2-} \rightarrow 2\,\text{I}^- + \text{S}_4\text{O}_6^{2-}$ . What is the percentage of copper in the brass by mass, to the nearest integer? Atomic masses in $\text{g mol}^{-1}$ : Cu 63.5.

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