A basic solution contains the iodide and phosphate ions that are to be separated

Question

A basic solution contains the iodide and phosphate ions that are to be separated via selective precipitation. The I– concentration, which is 8.30×10^-5 M, is 10,000 times less than that of the PO43– ion at 0.830 M . A solution containing the silver(I) ion is slowly added. Answer the questions below. Ksp of AgI is 8.30×10^-17 and of Ag3PO4, 8.90×10^-17. Calculate the minimum Ag+ concentration required to cause precipitation of AgI.____mol/L Calculate the minimum Ag+ concentration required to cause precipitation of Ag3PO4.____mol/L

Explanation / Answer

¡) Dissolution equillibrium of AgI

AgI(s) <--------> Ag+(aq) + I-(aq)

Ksp = [Ag+] [I-]= 8.30×10^-17

given concentration of I- =8.30×10^-5M

Therefore,

[Ag+]×8.30×10^-5M = 8.30×10^-17M^2

[ Ag+ ] = 1×10^-12M

Therefore,

Minimum concentration of Ag+ required to cause precipitation of AgI is 1×10^-12M

ii) Dissociation equillibrium of Ag3PO4 is

Ag3PO4(s) --------> 3Ag+(aq) + PO43-

Ksp = [ Ag+ ]^3 [ PO43- ] = 8.90×10^-17

given concentration of PO43- = 0.830M

So,

[ Ag+ ]^3 × 0.830M = 8.90×10^-17M^4

[ Ag+ ]^3 = 1.07×10^-16M^3

[ Ag+ ] = 4.75 × 10^-6M

Therefore,

the minimum Ag+ concentration to cause precipitation of Ag3PO4 is 4.75×10^-6M

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