A sample of hard water contains about 2.0 ×10^-3 M Ca^2+. A soluble fluoridecontaining salt such

step 1 To figure out the concentration of fluoride that will initiate precipitation of calcium fluoride

A sample of hard water contains about 2.0 ×10^-3 M Ca^2+. A...

A sample of hard water contains about $2.0 \times 10^{-3} \mathrm{M} \mathrm{Ca}^{2+}$. A soluble fluoridecontaining salt such as $\mathrm{NaF}$ is added to fluoridate the water (to aid in the prevention of dental cavities). What is the maximum concentration of $\mathrm{F}^{-}$that can be present without precipitating $\mathrm{CaF}_2$ ?

Key Concepts

Solubility Product (Ksp)

The solubility product is a constant that expresses the equilibrium between a sparingly soluble ionic compound and its ions in a saturated solution. It is calculated by multiplying the concentrations of the constituent ions, each raised to the power of its coefficient in the dissolution equation. This constant determines the maximum product of ion concentrations that can exist in solution without precipitation occurring.

Ionic Equilibrium

Ionic equilibrium involves the balance between the dissolution of a solid ionic compound and the precipitation of its constituent ions in solution. It is governed by principles of dynamic equilibrium where changes in ion concentrations can shift the balance, thus influencing the solubility of the compound.

Precipitation Reaction

A precipitation reaction occurs when the product of the concentrations of the ions in solution exceeds the solubility product (Ksp), leading to the formation of an insoluble compound. Determining the conditions under which precipitation occurs is crucial for controlling the concentration of ions in various applications.

Common Ion Effect

The common ion effect describes the phenomenon where the presence of a common ion suppresses the solubility of a sparingly soluble salt. When an additional source of one of the ions is added to the solution, it shifts the ionic equilibrium, often resulting in decreased solubility and influencing the threshold for precipitation.

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