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Chemistry · General chemistry II · Worked example

Find Ksp from a measured molar solubility

A saturated solution of calcium fluoride contains 2.1 × 10⁻⁴ mol of dissolved CaF₂ per liter. Find Ksp.

Find the ion concentrations

Each CaF₂ that dissolves gives one Ca²⁺ and two F⁻: [Ca²⁺] = 2.1 × 10⁻⁴ M and [F⁻] = 4.2 × 10⁻⁴ M.

Substitute into Ksp

Square the fluoride concentration, as its coefficient requires.

Ks⁢p=[Ca2+]⁢[F−]2=(2.1×10−4)⁢(4.2×10−4)2=3.7×10−11
Ks⁢p=[Ca2+]⁢[F−]2=(2.1×10−4)⁢(4.2×10−4)2=3.7×10−11

Watch the coefficient

Writing [F⁻] = S instead of 2S gives S³ = 9.3 × 10⁻¹², four times too small.

Result

Ksp = 3.7 × 10⁻¹¹.

Your turn

Silver chloride dissolves to 1.34 × 10⁻⁵ M. Find Ksp.

Show the answer and explanation

Ksp = 1.80 × 10⁻¹⁰.

[Ag⁺] = [Cl⁻] = 1.34 × 10⁻⁵ M, so Ksp = (1.34 × 10⁻⁵)² = 1.80 × 10⁻¹⁰.

Ks⁢p=(1.34×10−5)2=1.80×10−10

Keep exploring

In Solubility & precipitation, go the other way: with Ksp = 3.7 × 10⁻¹¹, the molar solubility comes back as 2.1 × 10⁻⁴ M.

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