**Step 1: ****Calculate the I ^{-} needed to precipitate AgI and PbI_{2}.**

**For AgI**

Calculate the **concentration of I ^{–}** needed in order to precipitate

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

AgI(s) ⇌ **Ag ^{+}(aq)** + I

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

The **K _{sp} expression** for AgI is:

$\overline{){{\mathbf{K}}}_{{\mathbf{sp}}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}\frac{\mathbf{products}}{\overline{)\mathbf{reactants}}}{\mathbf{=}}\mathbf{\left[}{\mathbf{Ag}}^{\mathbf{+}}\mathbf{\right]}\mathbf{\left[}{\mathbf{I}}^{\mathbf{-}}\mathbf{\right]}}$

*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**.

${\mathbf{K}}_{\mathbf{sp}}\mathbf{}\mathbf{=}\mathbf{\left[}{\mathbf{Ag}}^{\mathbf{+}}\mathbf{\right]}\mathbf{\left[}{\mathbf{I}}^{\mathbf{-}}\mathbf{\right]}\mathbf{}\mathbf{=}\mathbf{}\mathbf{8}\mathbf{.}\mathbf{3}\mathbf{\times}{\mathbf{10}}^{\mathbf{-}\mathbf{17}}\phantom{\rule{0ex}{0ex}}\mathbf{[}\mathbf{2}\mathbf{\times}{\mathbf{10}}^{\mathbf{-}\mathbf{4}}\mathbf{]}\mathbf{\left[}\mathbf{x}\mathbf{\right]}\mathbf{}\mathbf{=}\mathbf{}\mathbf{8}\mathbf{.}\mathbf{3}\mathbf{\times}{\mathbf{10}}^{\mathbf{-}\mathbf{17}}\phantom{\rule{0ex}{0ex}}\mathbf{x}\mathbf{}\mathbf{=}\mathbf{}\mathbf{}\frac{\mathbf{8}\mathbf{.}\mathbf{3}\mathbf{\times}{\mathbf{10}}^{\mathbf{-}\mathbf{17}}}{\mathbf{2}\mathbf{\times}{\mathbf{10}}^{\mathbf{-}\mathbf{4}}}$

x = 4.15×10^{-13 }M = [I^{-}]

A solution contains 2.0 x 10^{-4 }M Ag^{+} and 1.5 x 10^{-3} M Pb^{2+}. 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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