Simultaneous Equilibria

Question

In a solution containing both AgCl and AgBr, if Ksp(AgCl) = 10⁻¹⁰ and Ksp(AgBr) = 10⁻¹³, which compound will precipitate first when Ag⁺ ions are added?

RankGuru · Accepted Answer

1. **Precipitation Condition**: For a salt $XY$ to precipitate, the ionic product must exceed the solubility product ($I.P. \ge K_{sp}$).
2. **Analysis**: Assume equal concentrations of $Cl^-$ and $Br^-$.
- To precipitate $AgCl$, $[Ag^+] \ge \frac{10^{-10}}{[Cl^-]}$
- To precipitate $AgBr$, $[Ag^+] \ge \frac{10^{-13}}{[Br^-]}$
3. **Conclusion**: Since $10^{-13}$ is a much smaller requirement for $[Ag^+]$ than $10^{-10}$, the concentration of silver ions will reach the saturation point of **$AgBr$** first.
- The salt with the **lower $K_{sp}$** (for identical stoichiometry) precipitates first.

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Simultaneous Equilibria

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