An aqueous solution of sodium fluoride is slowly added to a water sample that co

Question

An aqueous solution of sodium fluoride is slowly added to a water sample that contains barium ion (0.0360 M) and calcium ion (0.0330 M) The K_ap of barium fluoride is 1.0 Times 10^-6. The K_sp of calcium ftuoride is 3.9 Times10^-11. What is the remaining concentration of the first ion to precipitate when the second ion begins to precipitate? Since K_sp of calcium fluoride is smaller than that of barium fluoride, the calcium fluoride will precipitate first. The K-sp reaction for calcium fluoride is: CuF_2(s) Ca^2+ (aq) + 2 F^- (aq) How would you determine when the second ion begins to precipitate? There can only be one fluoride concentration in the solution at any point in time. The same fluoride concentration applies to both solubility products.

Explanation / Answer

Start with more soluble salt,

Ksp for BaF2 = [Ba2+][F-]^2

1 x 10^-6 = (0.036)[F-]^2

[F-] = 5.27 x 10^-3 M

This is F- concentration when second cation begins to precipitate

What is the [Ca2+] concentration in solution at this F- concentration,

Ksp = [Ca2+][F-]^2

3.9 x 10^-11 = [Ca2+](5.27 x 10^-3)^2

[Ca2+] = 1.40 x 10^-6 M is the remaining concentration of first ion

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