Write the ion-product expressions for (a) lead(II) iodide; (b) strontium sulfate; (c) chromium(III) hydroxide.

Understanding the solubility product constant (Ksp) is key to grasping how salts dissolve in water. The Ksp value represents the level at which a solute dissolves in solution. Higher Ksp values mean greater solubility. To determine Ksp, you need the concentrations of the ions produced when a compound dissolves.
For example, for lead(II) iodide (PbI2), the dissociation produces lead ions (Pb²⁺) and iodide ions (I⁻). The Ksp expression is: \[ K_{sp} = [Pb^{2+}] [I^{-}]^2 \] The Ksp equation helps predict whether a precipitate will form when solutions are mixed. If the ion product exceeds the Ksp, precipitation occurs. Otherwise, the solution remains unsaturated.

Dissociation equations describe how ionic compounds split into their ions in aqueous solutions. These equations are essential for writing ion-product expressions.
For instance, when lead(II) iodide (PbI2) dissolves, the equation is: \[ PbI_2(s) \rightarrow Pb^{2+}(aq) + 2I^{-}(aq) \] Similarly, strontium sulfate (SrSO4) dissociates as: \[ SrSO_4(s) \rightarrow Sr^{2+}(aq) + SO_4^{2-}(aq) \] And chromium(III) hydroxide (Cr(OH)3) dissociates as: \[ Cr(OH)_3(s) \rightarrow Cr^{3+}(aq) + 3OH^{-}(aq) \] These equations show the ratio at which ions are produced, which is crucial for calculating the Ksp.

Chemical equilibria refer to the state where the concentrations of reactants and products remain constant over time. This is critical in understanding how dissolution and precipitation reactions balance.
When a salt dissolves, it reaches a dynamic equilibrium where the rate of dissolution equals the rate of precipitation. For example, in lead(II) iodide: \[ PbI_2(s) \rightleftharpoons Pb^{2+}(aq) + 2I^{-}(aq) \] The Ksp expression derived from this equilibrium helps predict the concentrations of ions in solution. If an additional substance alters the ion concentrations, the system adjusts to maintain equilibrium.
Understanding equilibria ensures you can predict changes in solubility or when a precipitate will form, based on ion concentrations and other factors.