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Question:

Solid BaCO₃ is gradually dissolved in a 1.0 × 10⁻⁸ M Na₂CO₃ solution. At what concentration of Ba²⁺ will a precipitate form?

4.1 × 10⁻⁹ M

5.1 × 10⁻⁹ M

8.1 × 10⁻¹¹ M

8.1 × 10⁻¹² M

Solution:

Na₂CO₃ → 2Na⁺ + CO₃²⁻
[CO₃²⁻] = 1 × 10⁻⁸ M
BaCO₃ → Ba²⁺ + CO₃²⁻
Ksp = [Ba²⁺][CO₃²⁻]
5.1 × 10⁻⁹ = [Ba²⁺][1 × 10⁻⁸]
[Ba²⁺] = 5.1 × 10⁻⁹ / 1 × 10⁻⁸ = 5.1 × 10⁻¹ M