Question

A solution contains 1.46×10-2 M sodium fluoride and 1.48×10-2 M potassium carbonate. Solid magnesium acetate is added slowly to this mixture. What is the concentration of fluoride ion when carbonate ion begins to precipitate? [fluoride] = M

Answer 1

[fluoride] = 1.54 x 10^-3 M

Explanation

initial concentration of carbonate = 1.48 x 10^-2 M

Ksp MgCO₃ = 4.0 x 10^-5

Concentration of Mg²⁺ when carbonate precipitates = (Ksp MgCO₃) / (initial concentration of carbonate)

Concentration of Mg²⁺ when carbonate precipitates = (4.0 x 10^-5) / (1.48 x 10^-2 M)

Concentration of Mg²⁺ when carbonate precipitates = 2.70 x 10^-3 M

Ksp MgF₂ = 6.4 x 10^-9

Concentration of F^- when carbonate precipitates = [(Ksp MgF₂) / (Concentration of Mg²⁺ when carbonate precipitates)]^1/2

Concentration of F^- when carbonate precipitates = [(6.4 x 10^-9) / (2.70 x 10^-3 M)]^1/2

Concentration of F^- when carbonate precipitates = (2.37 x 10^-6 M²)^1/2

Concentration of F^- when carbonate precipitates = 1.54 x 10^-3 M