Criteria for Precipitation and Its Completeness

Silver iodide is a light-sensitive compound used in photographic film and also in cloud seeding to produce rain. Its solubility equilibrium and K p are represented as 

Suppose we mix solutions of AgNOsfaq) and Kl(aq) to obtain a mixed solution that has [Ag ] = 0.010 M and [I ] = 0.015 M. Is this solution unsaturated, saturated, or supersaturated  

Recall the reaction quotient, Q, that was described in detail in Chapter 15. It has the same form as an equilibrium constant expression but uses initial concentrations rather than equilibrium concentrations. Initially, 

The fact that Qsp K p indicates that the concentrations of Ag and 1 are already higher than they would be in a saturated solution and that a net change should occur to the left. The solution is supersaturated. As is generally the case with supersaturated solutions, excess Agl should precipitate from solution. If we had found Q, 

No precipitate would form from such a solution. 

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Criteria for Precipitation and Its Completeness 18-6 Fractional Precipitation 18-7 Solubility and pH 18-8 Equilibria Involving Complex Ions 18-9 Qualitative Cation Analysis... 

Criteria for Precipitation and Its Completeness—To determine whether a slightly soluble solute will precipitate from a solution, the ion product, Qsp, is compared with the solubility product constant, K p. Q p is based on the initial ion concentrations in a solution. K p, on the other hand, is based on the equilibrium ion concentrations in a saturated solution. If Qgp > K p, precipitation will... 

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