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17.7: Solubility and Complex Ions

  • Page ID
    60767
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    Introduction

    Many cations form both precipitates and complex ions, and sometimes with the same ion. For example silver forms both a precipitate and a complex ion with chloride, and so the solubility product alone will lead to an incorrect estimate of its solubility.

    Solubility Product for AgCl:

    \[ AgCl(s) \leftrightharpoons Ag^+(aq) + Cl^-(aq) \; \; \; \; K_{sp}=1.8x10^{-10}\]

    Complex ion formation for Silver and chloride:

    \[Ag^+(aq) + 2Cl^-(aq) \leftrightharpoons [AgCl_2]^- \; \; \; \; K_f= 1.1x10^5\]

    For pure silver chloride, Ksp gives the silver and chloride concentrations for a saturated solution because the ratio of silver to chloride is one-to-one. If an external source of chloride was introduced via another salt where chloride was a common ion the solubility of the silver does not go down as predicted by the common ion effect and Le Chatlier's Principle, but it actually goes up due to the formation of a complex ion. In the case where a second salt contributes chloride we need to add the above two equations, which means the total equilibrium constant is the product of the above two (review section 15.5, Balanced Equations and Equilibrium Constants).

    \[AgCl(s) \leftrightharpoons Ag^+(aq) + Cl^-(aq) \; \; \; \; K = K_{sp} \\ \underline{ Ag^+(aq) + 2Cl^-(aq) \leftrightharpoons [AgCl_2]^-(aq)} \; \; \; \; K = K_f \\ AgCl(s)+Cl^-(aq) \leftrightharpoons [AgCl_2]^-(aq) ;\; \; \; K=K_{sp}K_f = 2.0x10^{-5} \]

    Exercise \(\PageIndex{1}\)

    What is the [AgCl2]- concentration in the presences of AgCl(s) for a solution that is 1.0M NaCl?

    Answer

    \(K=\dfrac{[\mathrm{AgCl_2^-}]}{[\mathrm{Cl^-}]}=\dfrac{x}{1.0-x}\approx 2\times10^{-5}=x\)

    Contributors and Attributions

    Robert E. Belford (University of Arkansas Little Rock; Department of Chemistry). The breadth, depth and veracity of this work is the responsibility of Robert E. Belford, rebelford@ualr.edu. You should contact him if you have any concerns. This material has both original contributions, and content built upon prior contributions of the LibreTexts Community and other resources, including but not limited to:


    This page titled 17.7: Solubility and Complex Ions is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Robert Belford.

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