An aqueous solution contains 10⁻² M Na2SO4 and 10⁻² M NaI. Pure Pb(NO3)2 is added gradually. What is the concentration of SO4²- ions when PbI2 just starts to precipitate? (Given: Ksp(PbI2) = 10⁻⁹ and Ksp(PbSO4) = 10⁻⁸)

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Accepted Answer

10⁻⁵ M. PbSO4 precipitates before PbI2 (check: PbSO4 precipitates when [Pb²+] = 10⁻⁸/10⁻² = 10⁻⁶ M; PbI2 precipitates when [Pb²+] = 10⁻⁹/(10⁻²)² = 10⁻⁵ M). So PbSO4 precipitates first (lower [Pb²+] needed). When PbI2 just starts to precipitate: [Pb²+] = 10⁻⁵ M. At this point, [SO4²-] = Ksp(PbSO4)/[Pb²+] = 10⁻⁸/10⁻⁵ = 10⁻³ M.

An aqueous solution contains 10⁻² M Na2SO4 and 10⁻² M NaI. Pure Pb(NO3)2 is added gradually. What is the concentration of SO4²- ions when PbI2 just starts to precipitate? (Given: Ksp(PbI2) = 10⁻⁹ and Ksp(PbSO4) = 10⁻⁸)

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