Question

# A solution of NaF is added dropwise to a solution that is 0.0532 M in Mg2+...

A solution of NaF is added dropwise to a solution that is 0.0532 M in Mg2+ and 1.43e-05 M in Y3+.

The Ksp of MgF2 is 5.16e-11.
The Ksp of YF3 is 8.62e-21.

(a) What concentration of F- is necessary to begin precipitation? (Neglect volume changes.)

[F-] =  M.

(b) Which cation precipitates first?

Mg2+Y3+

(c) What is the concentration of F- when the second cation begins to precipitate?

[F-] =  M.

a) Magnesium fluoride precipitate according to
Mg2+(aq) + 2 F⁻(aq) ←→ MgF2(s)

When MgF2 precipitates ion concentrations satisfy solubility product:
Ksp = [Mg2+] * [F-]2

Assuming NaF is concentrated enough, that only a small portion of solution is added till precipitation, the change of [Mg2+] due to dilution may be ignored. Hence:    b) Mg2+ will be precipitate first than Y3+

c) Yttrium fluoride precipitate according to
Y3+(aq) + 3 F⁻(aq) ←→ YF3(s)

When YF3 precipitates ion concentrations satisfy solubility product:
Ksp = [Y3+] * [F-]3

Assuming NaF is concentrated enough, that only a small portion of solution is added till precipitation, the change of [Y3+] due to dilution may be ignored. Hence:    #### Earn Coins

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