A 2.54 g sample of a copper-containing alloy is dissolved in acid and the resulting solution is diluted to a total volume of 250 mL . A 25 mL aliquot of this solution is treated with an excess of KI , and the liberated iodine requires 20 mL of 0.1 M sodium thiosulphate solution for complete reaction. Assuming no other components of the alloy react with KI , the percentage by mass of copper in the alloy is ________. (

Step-by-step solution

The reactions involved are: 2 Cu ²⁺ + 4 I ^- Cu ₂ I ₂ + I ₂ I ₂ + 2 S ₂ O ₃²⁻ 2 I ^- + S ₄ O ₆²⁻ Millimoles of S ₂ O ₃²⁻ used in titration = 20 0.1 = 2.0 mmol . Millimoles of I ₂ liberated = 2.0 2 = 1.0 mmol . From the first reaction, 2 moles of Cu ²⁺ produce 1 mole of I ₂ . Millimoles of Cu ²⁺ in the 25 mL aliquot = 1.0 2 = 2.0 mmol . Total millimoles of Cu ²⁺ in the 250 mL solution = 2.0 ( 250 25 ) = 20 mmol . Mass of copper in the alloy = 20 10⁻³ mol 63.5 g/mol = 1.27 g . Percentage of copper in the alloy = ( 1.