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.
Ch.17 - Applications of Aqueous Equilibria
Chapter 17, Problem 135
Will CoS precipitate in a solution that is 0.10 M in Co(NO3)2, 0.5 M in HCl, and 0.10 M in H2S? Will CoS precipitate if the pH of the solution is adjusted to pH 8 with an NH4+ - NH3 buffer? Ksp for CoS is 3.
Verified step by step guidance1
Step 1: Identify the relevant chemical reaction for the precipitation of CoS. The reaction is: Co^{2+} + S^{2-} \rightleftharpoons CoS(s).
Step 2: Calculate the concentration of sulfide ions (S^{2-}) in the solution. In acidic conditions, H2S is only partially dissociated, so use the dissociation equilibrium of H2S: H2S \rightleftharpoons H^+ + HS^- \rightleftharpoons 2H^+ + S^{2-}.
Step 3: Determine the concentration of S^{2-} in the acidic solution using the given concentration of HCl (0.5 M) and the initial concentration of H2S (0.10 M). Consider the effect of the strong acid (HCl) on the dissociation of H2S.
Step 4: Calculate the ion product (Q) for CoS using the concentrations of Co^{2+} (0.10 M) and the calculated S^{2-} concentration. Compare Q to the Ksp of CoS (3 \times 10^{-21}) to determine if precipitation occurs.
Step 5: For the solution at pH 8, calculate the new concentration of S^{2-} using the NH4^+ - NH3 buffer. Use the Henderson-Hasselbalch equation to find the ratio of NH3 to NH4^+ and adjust the S^{2-} concentration accordingly. Recalculate Q and compare it to Ksp to determine if CoS will precipitate at pH 8.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
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 product of the molar concentrations of the ions in a saturated solution at a given temperature. For CoS, Ksp = [Co^2+][S^2-], and if the product of the ion concentrations exceeds Ksp, precipitation occurs.
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Solubility Product Constant
Common Ion Effect
The common ion effect describes the decrease in solubility of an ionic compound when a common ion is added to the solution. In this case, the presence of Co^2+ from Co(NO3)2 can shift the equilibrium of CoS dissolution, potentially leading to precipitation. Understanding this effect is crucial for predicting whether CoS will precipitate under varying conditions.
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pH and Precipitation
The pH of a solution significantly influences the solubility of sulfide compounds like CoS. At lower pH levels, the concentration of H+ ions can suppress the formation of sulfide ions (S^2-), reducing the likelihood of precipitation. Conversely, increasing the pH to 8 with an NH4+-NH3 buffer raises the concentration of S^2-, which may lead to precipitation if the product of ion concentrations exceeds Ksp.
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Related Practice
Textbook Question
Will FeS precipitate in a solution that is 0.10 M in Fe(NO3)2, 0.4 M in HCl, and 0.10 M in H2S? Will FeS precipitate if the pH of the solution is adjusted to pH 8 with an NH4+ - NH3 buffer? Kspa = 6 x 10^2 for FeS.
Textbook Question
Using the qualitative analysis flowchart in Figure 17.18 , tell how you could separate the following pairs of ions. (a) Ag+ and Cu2+
Textbook Question
Give a method for seperating the following pairs of ions by the addition of no more than two substances. (a) Hg2+ and Co2+
Textbook Question
Assume that you have three white solids: NaCl, KCl, and MgCl2. What tests could you do to tell which is which?