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 7.80×10-5 M, is 10,000 times less than that of the PO43– ion at 0.780 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.

Calculate the minimum Ag+ concentration required to cause precipitation of Ag3PO4.

Explanation / Answer

For the precipitation of AgI remember the ksp definition

AgI ========= Ag+ + I-

Ksp = [Ag+] [I-] = 8.30×10-17

8.30×10-17 =  7.80×10-5 * [I]

[I] = 8.30×10-17 / 7.80×10-5 = 1.064 x 10-12

Ag3PO4 ---> 3 Ag+ + PO43-, here we have a 3 stoichiometrical coefficient so we must consider it in the ksp equation

Ksp = [Ag+]3[PO43-] = 8.9 x 10-17

Ksp = [Ag+]3[0.78] = 8.9 x 10-17

[Ag+]3 = 8.9 x 10-17 / 0.78 = 1.141 x 10-6

[Ag] = (1.141 x 10-6)1/3 = 4.85 x 10-6

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