Problem: A solution contains 2.0 x 10-4 M Ag+ and 1.5 x 10-3 M Pb2+. If NaI is added, will AgI (Ksp = 8.3 x 10-17) or PbI2 (Ksp = 7.9 x 10-9) precipitate first? Specify the concentration of I- needed to begin precipitation.

FREE Expert Solution

Step 1: Calculate the I- needed to precipitate AgI and PbI2.


For AgI

Calculate the concentration of I needed in order to precipitate Ag+.


The compound is an ionic compound, it forms ions when dissociating in water. The dissociation of AgI is:

AgI(s)  Ag+(aq) + I(aq)


We can construct an ICE table for the dissociation of AgI. Remember that solids are ignored in the ICE table.



The Ksp expression for AgI is:


Ksp = productsreactants=[Ag+][I-]

Solids and liquids are ignored in the expression.


Note that each concentration is raised by the stoichiometric coefficient: both [Ag+] and [I] are raised to 1.


We can now plug in the equilibrium concentrations and solve for x, which is the molar concentration of I needed to start precipitation or the molar solubility of AgI.


Ksp =[Ag+][I-] = 8.3×10-17[2×10-4][x] = 8.3×10-17x =  8.3×10-172×10-4

x = 4.15×10-13 M = [I-]


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Problem Details

A solution contains 2.0 x 10-4 M Ag+ and 1.5 x 10-3 M Pb2+. If NaI is added, will AgI (Ksp = 8.3 x 10-17) or PbI2 (Ksp = 7.9 x 10-9) precipitate first? Specify the concentration of I- needed to begin precipitation.

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