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Ch.18 - Chemistry of the Environment
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.18 - Chemistry of the EnvironmentProblema 75b
Capitolo 18, Problema 75b

(b) Will Mg(OH)2 precipitate when 4.0 g of Na2CO3 is added to 1.00 L of a solution containing 125 ppm of Mg2+?

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Convert the concentration of Mg^{2+} from ppm to molarity. Since 125 ppm is equivalent to 125 mg/L, convert this to moles by dividing by the molar mass of Mg (24.31 g/mol).
Calculate the moles of Na2CO3 added by converting 4.0 g of Na2CO3 to moles using its molar mass (105.99 g/mol).
Determine the concentration of CO3^{2-} ions in the solution after adding Na2CO3. Since the volume of the solution is 1.00 L, the concentration is equal to the moles of Na2CO3.
Use the solubility product constant (K_{sp}) for Mg(OH)2 to determine the ion product (Q) of Mg^{2+} and OH^{-} in the solution. The K_{sp} expression for Mg(OH)2 is K_{sp} = [Mg^{2+}][OH^{-}]^2.
Compare the ion product (Q) to the K_{sp} value for Mg(OH)2. If Q > K_{sp}, Mg(OH)2 will precipitate; if Q < K_{sp}, it will not.

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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 Mg(OH)2, Ksp indicates the maximum concentration of Mg2+ and OH- ions that can exist in solution before precipitation occurs. Understanding Ksp is essential to determine if the concentration of ions exceeds this value, leading to precipitation.
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Solubility Product Constant

Precipitation Reactions

Precipitation reactions occur when two soluble salts react to form an insoluble compound, or precipitate. In this case, adding Na2CO3 introduces carbonate ions, which can react with Mg2+ ions to form Mg(OH)2. Recognizing the conditions under which precipitation occurs is crucial for predicting whether a solid will form in a solution.
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Selective Precipitation

Concentration Calculations

Concentration calculations involve determining the molarity of ions in a solution, which is essential for assessing whether a precipitate will form. In this scenario, converting the mass of Na2CO3 to moles and then calculating the resulting concentration of Mg2+ ions allows for comparison with Ksp. Accurate concentration calculations are vital for predicting the outcome of the reaction.
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