Question

Both AgCl and AgI are very sparingly soluble salts, but the solubility of AgI is much less than that of AgCl, as can be seen by their Ksp values. Suppose that a solution contains both Cl− and I− with [Cl−] = 0.046 M and [I−] = 0.047 M. If solid AgNO3 is added to 2.22 L of this mixture (so that no appreciable change in volume occurs), what is the value of [I−] when AgCl first begins to precipitate?

Answer 1

CONCEPT: The solubility (or Ksp) of AgI is very small compared to AgCl. It means that when Ag+ (AgNO3) is added to the mixture of Cl^- and I^- ions AgI will precipitate first. After that when more Ag+ is added Agl will begin to precipitate. We have to measure the concentration of Ag+ where AgCl will begin tpprecipitate. Thiscan be calculated fro Ksp equation. After that we can easily calculate the remaining I- ions in solution from same equation.

The solution contains 0.046M Cl- ions.These ions will react with Ag+ from AgNO3 to form ppt of AgCl. AgCl ppt will form only after a minimum concentration of Ag+ given blow

Ag⁺ + + Cl^- <------> AgCl

Ksp = [Cl-][Ag+]

[Cl-] = 0.046 Ksp = 1.8X 10^-10

[Ag+] = 1.8X 10^-10 / 0.046 = 39.13 X 10^-10

concntration of I- can be calculted from Ksp equation when Ag⁺ is 39.13 X 10^-10

8.52 X 10^-17= [I-][39.13 X 10^-10 ]

[I^-] = 8.52/39.13 X 10^-10 = 2.17 X 10^-8