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Ch.17 - Additional Aspects of Aqueous Equilibria
Brown - Chemistry: The Central Science 14th Edition
Brown14th EditionChemistry: The Central ScienceISBN: 9780134414232Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoBrown 14th EditionCh.17 - Additional Aspects of Aqueous EquilibriaProblema 118
Capitolo 17, Problema 118

Fluoridation of drinking water is employed in many places to aid in the prevention of tooth decay. Typically. the Fion concentration is adjusted to about 1 ppm. Some water supplies are also 'hard'; that is, they contain certain cations such as Ca2 + that interfere with the action of soap. Consider a case where the concentration of Ca2 + is 8 ppm. Could a precipitate of CaF2 form under these conditions? (Make any necessary approximations.)

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Convert the concentration of fluoride ions (F^-) from ppm to molarity. Assume the density of water is approximately 1 g/mL, so 1 ppm is equivalent to 1 mg/L.
Convert the concentration of calcium ions (Ca^{2+}) from ppm to molarity using the same assumption about water density.
Write the solubility product expression for calcium fluoride (CaF_2), which is K_{sp} = [Ca^{2+}][F^-]^2.
Use the molarity values obtained for [Ca^{2+}] and [F^-] to calculate the ion product, Q = [Ca^{2+}][F^-]^2.
Compare the ion product, Q, to the known solubility product constant, K_{sp}, for CaF_2. If Q > K_{sp}, a precipitate will form; if Q < K_{sp}, no precipitate will form.

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Solubility Product Constant (Ksp)

The solubility product constant (Ksp) is a numerical value that represents the equilibrium between a solid and its ions in a saturated solution. For calcium fluoride (CaF2), Ksp can be used to determine whether a precipitate will form when the concentrations of its constituent ions exceed the Ksp value. Understanding Ksp is crucial for predicting precipitation reactions in solutions.
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Solubility Product Constant

Ion Concentration and Precipitation

The concentration of ions in a solution directly influences the likelihood of precipitation. When the product of the concentrations of the ions in solution exceeds the Ksp, a precipitate will form. In this case, the concentrations of Ca2+ and F- ions must be calculated to assess whether CaF2 will precipitate when Ca2+ is present at 8 ppm and F- at 1 ppm.
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Selective Precipitation

Hard Water and Its Effects

Hard water contains high concentrations of cations such as Ca2+ and Mg2+, which can interfere with various chemical processes, including soap effectiveness and precipitation reactions. The presence of Ca2+ in the water can affect the solubility of compounds like CaF2, making it essential to consider the hardness of the water when evaluating potential precipitate formation.
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