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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
Capitolo 18, Problema 90a

The precipitation of Al(OH)3 (Ksp) = 1.3⨉10-33) is sometimes used ot purify water. (a) Estimate the pH at which precipitation of Al(OH)3 will begin if 5.0 lb of Al2(SO4)3 is added to 2000 gal of water

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1
Convert the mass of Al_2(SO_4)_3 from pounds to grams using the conversion factor 1 lb = 453.592 g.
Calculate the number of moles of Al_2(SO_4)_3 using its molar mass (Al_2(SO_4)_3 has a molar mass of approximately 342.15 g/mol).
Determine the concentration of Al^3+ ions in the solution by dividing the moles of Al_2(SO_4)_3 by the volume of water in liters (1 gal = 3.78541 L).
Use the K_sp expression for Al(OH)_3, which is K_sp = [Al^3+][OH^-]^3, to find the concentration of OH^- needed for precipitation.
Convert the concentration of OH^- to pH using the relation pH + pOH = 14, where pOH = -log[OH^-].

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Solubility Product Constant (Ksp)

The solubility product constant (Ksp) is an equilibrium constant that applies to the solubility of sparingly soluble ionic compounds. It represents the maximum concentration of ions in a saturated solution at a given temperature. For Al(OH)3, Ksp = 1.3 × 10^-33 indicates that the compound is very insoluble, meaning that only a tiny amount can dissolve in water before precipitation occurs.
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01:47
Solubility Product Constant

pH and Hydroxide Ion Concentration

pH is a measure of the acidity or basicity of a solution, defined as the negative logarithm of the hydrogen ion concentration. In the context of Al(OH)3 precipitation, the pH must be sufficiently high to increase the concentration of hydroxide ions (OH-) in the solution. As the pH rises, the solubility of Al(OH)3 decreases, leading to precipitation when the product of the ion concentrations exceeds Ksp.
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Hydroxide Ion Concentration Example

Stoichiometry and Dilution

Stoichiometry involves the calculation of reactants and products in chemical reactions, while dilution refers to the process of reducing the concentration of a solute in a solution. In this question, understanding how to convert the mass of Al2(SO4)3 added to water into moles and then into hydroxide ion concentration is crucial for estimating the pH at which Al(OH)3 will begin to precipitate.
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Dilution Equation
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