Question

A 35.0-mL solution of 0.075 M CaCl2 is mixed with 25.0 mL of 0.090 M BaCl2.

If aqueous KF is added, which fluoride precipitates first?

Answer 1

The fluoride which precipitates first is CaF₂

Step-by-step explanation:

When F⁻ is added, CaF₂ and BaF₂ are produced following the ksp equation:

For CaF₂:

Ksp = 3.2x10⁻¹¹ = [Ca²⁺] [F⁻]²

Where [Ca²⁺] = 0.075M * {35mL / (25mL + 35mL)} = 0.04375M

3.2x10⁻¹¹ = [0.04375M] [F⁻]²

[F⁻]² = 7.31x10⁻¹⁰

[F⁻] = 2.7x10⁻⁵M

CaF₂ begins precipitation when [F⁻] = 2.7x10⁻⁵M.

For BaF₂:

Ksp = 1.5x10⁻⁶ = [Ba²⁺] [F⁻]²

Where [Ba²⁺] = 0.090M * {25mL / (25mL + 35mL)} = 0.0375M

1.5x10⁻⁶ = [0.0375M] [F⁻]²

[F⁻]² = 4x10⁻⁵

[F⁻] = 6.3x10⁻³M

BaF₂ begins precipitation when [F⁻] = 6.3x10⁻³M

Thus, the fluoride which precipitates first is CaF₂