A solution of Na3AsO4 is added dropwise to a solution that is 0.0220 M in Cu2+ and 0.492 M in Ag+.
The Ksp of
Cu3(AsO4)2 is 7.95e-36.
The Ksp of Ag3AsO4 is
1.03e-22.
(a) What concentration of AsO43- is necessary
to begin precipitation? (Neglect volume changes.)
[AsO43-] = M.
(b) Which cation precipitates first?
Cu2+
Ag+
(c) What is the concentration of AsO43- when
the second cation begins to precipitate?
[AsO43-] = M.
a) Solubility equilibrium of Cu3(AsO4)2 is
Cu3(AsO4)2(s) <-------> 3Cu2+(aq) + 2AsO43-(aq)
Ksp = [Cu2+]3[AsO43-]2 = 7.95 ×10-36
substituting concentration of Cu2+ in Ksp expression
(0.0220M)3 × [AsO43-]2 = 7.95 ×10-36M5
[AsO43-]2 = 7.47 × 10-31M2
[AsO43-] = 8.64 ×10-16M
concentration of AsO43- required to start precipitation of Cu3(AsO4)2 = 8.64 ×10-16M
Solubility equilibrium of Ag3AsO4 is
Ag3AsO4(s) <--------> 3Ag+(aq) + AsO43-(aq)
Ksp = [Ag+]3[AsO43-] = 1.03 ×10-22
substitute the concentration of Ag+ in Ksp expression
( 0.492M)3 × AsO43- = 1.03 ×10-22M4
AsO43- = 8.65 × 10-22M
Therefore,
Concentration of AsO43- necessary to begin precipitation = 8.65 ×10-22M
b)
Concentration of AsO43- necessary to start precipitation of Ag3AsO4 is lower than concentration of AsO43- necessary to start precipitation of Cu3(AsO4)2
Therefore,
Ag+ precipitate first
c)
Substitute the concentration of AsO43 necessary to start precipitation of Cu3(AsO4)2in Ksp expression of Ag3AsO4
[Ag+]3 × [AsO43-] = 1.03 ×10-22
[Ag+]3 × ( 8.64 × 10-16M ) = 1.03 ×10-22M4
[Ag+]3 = 1.192× 10-7
[Ag+] = 4.92 × 10-3M
Therefore,
concentration of AsO43- when second cation begins to precipitate = 4.92 × 10-3M
A solution of Na3AsO4 is added dropwise to a solution that is 0.0220 M in Cu2+...
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