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The values of Ksp of CaCO3 and CaC2O4 are 4.7 × 10⁻⁹ and 1.3 × 10⁻⁹ respectively at 25°C. If the mixture of these two is washed with water, what is the concentration of Ca²⁺ ions in water?

1. 7.746 × 10⁻⁵ M
2. 5.831 × 10⁻⁵ M
3. 6.856 × 10⁻⁵ M
4. 3.606 × 10⁻⁵ M

Correct answer: 5.831 × 10⁻⁵ M

Solution

The solubility of Ca²⁺ ions is determined by the salt with the lower Ksp, as it will precipitate first. Since Ksp of CaC2O4 (1.3 × 10⁻⁹) is lower than that of CaCO3 (4.7 × 10⁻⁹), the concentration of Ca²⁺ ions is calculated using the Ksp of CaC2O4. Solving for [Ca²⁺] = √(Ksp) = √(1.3 × 10⁻⁹) ≈ 5.831 × 10⁻⁵ M.

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