AP Chemistrymediummcq1 pt
A solution contains 0.010 M Ca²⁺. What minimum fluoride concentration triggers precipitation of CaF₂? (Ksp = 3.9 × 10⁻¹¹)
A.3.9 × 10^-9 M
B.3.9 × 10^-11 M
C.6.2 × 10^-5 M
D.6.2 × 10^-4 M
Explanation
Core Concept
Precipitation begins the instant Q exceeds Ksp, so the boundary obeys [Ca²⁺][F⁻]² = 3.9 × 10⁻¹¹. Dividing by the fixed 0.010 M calcium gives [F⁻]² = 3.9 × 10⁻⁹. Taking the square root yields [F⁻] = 6.2 × 10⁻⁵ M. Above that level the solution is supersaturated and solid CaF₂ must form.
Correct Answer
C6.2 × 10^-5 M
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