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:
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