Superconductivity

Two identical samples of $\ce{YBa2Cu3O7_y}$ (YBCO) with an unknown value of $y$ were prepared. The first sample was dissolved in 5 mL of 1.0 M aqueous $\ce{ HCl}$ , evolving $\ce{O2}$ . After boiling to expel gases, cooling, and addition of 10 mL of 0.7 M $\ce{KI}$ solution under $\ce{Ar}$ , titration with thiosulfate to the starch endpoint required 1.542 x 10-4 mol thiosulfate. The second sample of YBCO was added directly to 7 mL of a solution that was 1.0 M in $\ce{KI }$ and 0.7 M in $\ce{HCl}$ under $\ce{Ar}$ ; titration of this solution required 1.696 x 10-4 mol thiosulfate to reach the endpoint. Calculate the value of $y$ for these samples of YBCO.