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Ch.17 - Applications of Aqueous Equilibria
All textbooksMcMurry 8th EditionCh.17 - Applications of Aqueous EquilibriaProblem 129
Chapter 17, Problem 129

The pH of a sample of hard water having [Mg2+] = 2.5 x 10^-4 M is adjusted to pH 10.80. Will Mg(OH)2 precipitate?

Verified step by step guidance
1
Step 1: Determine the solubility product constant (K_{sp}) for Mg(OH)_2. This value is typically found in a chemistry reference table. For Mg(OH)_2, K_{sp} is approximately 1.8 \times 10^{-11} at 25°C.
Step 2: Write the equilibrium expression for the dissolution of Mg(OH)_2: Mg(OH)_2 (s) \rightleftharpoons Mg^{2+} (aq) + 2OH^{-} (aq). The expression for K_{sp} is K_{sp} = [Mg^{2+}][OH^{-}]^2.
Step 3: Calculate the concentration of OH^{-} ions at pH 10.80. Use the relationship pH + pOH = 14 to find pOH, then calculate [OH^{-}] using [OH^{-}] = 10^{-pOH}.
Step 4: Calculate the ion product (Q) for Mg(OH)_2 using the concentrations: Q = [Mg^{2+}][OH^{-}]^2. Substitute the given [Mg^{2+}] = 2.5 \times 10^{-4} M and the calculated [OH^{-}] from Step 3.
Step 5: Compare the ion product (Q) to the solubility product constant (K_{sp}). If Q > K_{sp}, Mg(OH)_2 will precipitate. If Q < K_{sp}, no precipitation will occur.
Related Practice
Open Question
Zinc hydroxide, Zn(OH)2 (Ksp = 4.1 x 10^-17), is nearly insoluble in water but is more soluble in strong base because Zn2+ forms the soluble complex ion [Zn(OH)4]2- (Kf = 3 x 10^15). (a) What is the molar solubility of Zn(OH)2 in pure water? (You may ignore the OH- from the self-dissociation of water.) (b) What is the pH of the solution in part (a)? (c) What is the molar solubility of Zn(OH)2 in 0.10 M NaOH?
Textbook Question
Citric acid (H3Cit) can be used as a household cleaning agent to dissolve rust stains. The rust, represented as Fe(OH)3, dissolves because the citrate ion forms a soluble complex with Fe3+ (a) Using the equilibrium constants in Appendix C and Kf = 6.3 x 10^11 for Fe(Cit), calculate the equilibrium constant K for the reaction. (b) Calculate the molar solubility of Fe(OH)3 in 0.500 M solution of H3Cit.
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Open Question
Hard water contains alkaline earth cations such as Ca2+, which react with CO32- to form insoluble deposits of CaCO3. Will a precipitate of CaCO3 form if a 250 mL sample of hard water having [Ca2+] = 8.0 x 10^-4 M is treated with the following? (a) 0.10 mL of 2.0 x 10^-3 M Na2CO3 (b) 10 mg of solid Na2CO3
Open Question
In quantitative analysis, Al3+ and Mg2+ are separated in an NH4+ - NH3 buffer having pH approximately equal to 8. Assuming cation concentrations of 0.010 M, show why Al(OH)3 precipitates but Mg(OH)2 does not.
Textbook Question
Can Fe2+ be separated from Sn2+ by bubbling H2S through a 0.3 M HCl solution that contains 0.01 M Fe2+ and 0.01 M Sn2+? A saturated solution of H2S has [H2S] ≈ 0.10 M. Values of Kspa are 6 x 10^2 for FeS andd 1 x 10^-5 for SnS.
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Open Question
Can Co2+ be separated from Zn2+ by bubbling H2S through a 0.3 M HCl solution that contains 0.01 M Co2+ and 0.011 M Zn2+? A saturated solution of H2S has [H2S] approximately equal to 0.10 M. Values of Ksp are 3 for CoS and 3 x 10^-2 for ZnS.