Passage
Copper plays a vital role as a trace mineral for biological processes. The amount of dietary copper needed by an adult each day is about 30 μg/kg of body weight, and blood plasma normally has a copper concentration of 1 mg/L.Copper is an essential element, and toxic effects are not common. However, given sufficient exposure to high levels by inhalation, absorption, or ingestion from sources such as industrial hazards or contaminated groundwater, copper toxicity can occur.Salts that are more soluble tend to exhibit higher toxicity when ingested, but the solubilities of copper salts vary and are also temperature-dependent, as shown by selected examples in Figure 1.
Figure 1 The solubility of selected copper salts in distilled water with increasing temperatureSome copper salts have solubilities much lower than those shown in Figure 1. For example, CuF₂ has a solubility of only 0.075 g per 100 g of water (at 25 °C) . This indicates that CuF₂ dissociates in water only to a small extent (Reaction 1) and that the equilibrium favors the undissociated salt, giving CuF₂ a solubility product constant of K_sp = 1.6 × 10⁻⁶.
Reaction 1Compound-specific properties can also impact solubility. In CuF₂, the dissociated F⁻ ion is mildly basic (pK_b = 10.8) , causing the equilibrium to be influenced by the pH of the solution.Specific characteristics of copper can also play a role. For example, copper ions in solution express an affinity to form coordinate bonds with available lone-pair electrons of nitrogen atoms in amines (Reaction 2) .
Reaction 2If excess amine ligands are present, coordinate bonds like that shown in Reaction 2 will continue to form until each Cu²⁺ ion in solution is coordinated with four nitrogen atoms. This acquisition of copper ions by ligands in solution can also impact the solubility equilibrium of copper salts by forming soluble coordination complexes.
-For the separation of a saturated aqueous mixture of CuF₂ (K_sp = 1.6 × 10⁻⁶) and BaF₂ (K_sp = 3.0 × 10⁻⁶) , a lab technician chose to use the common ion effect to precipitate CuF₂ from the solution rather than BaF₂. Which of the following statements does NOT explain why precipitating CuF₂ is the better choice for the separation procedure?

Question

Passage
Copper plays a vital role as a trace mineral for biological processes. The amount of dietary copper needed by an adult each day is about 30 μg/kg of body weight, and blood plasma normally has a copper concentration of 1 mg/L.Copper is an essential element, and toxic effects are not common. However, given sufficient exposure to high levels by inhalation, absorption, or ingestion from sources such as industrial hazards or contaminated groundwater, copper toxicity can occur.Salts that are more soluble tend to exhibit higher toxicity when ingested, but the solubilities of copper salts vary and are also temperature-dependent, as shown by selected examples in Figure 1.

Figure 1 The solubility of selected copper salts in distilled water with increasing temperatureSome copper salts have solubilities much lower than those shown in Figure 1. For example, CuF₂ has a solubility of only 0.075 g per 100 g of water (at 25 °C) . This indicates that CuF₂ dissociates in water only to a small extent (Reaction 1) and that the equilibrium favors the undissociated salt, giving CuF₂ a solubility product constant of K_sp = 1.6 × 10⁻⁶.

Reaction 1Compound-specific properties can also impact solubility. In CuF₂, the dissociated F⁻ ion is mildly basic (pK_b = 10.8) , causing the equilibrium to be influenced by the pH of the solution.Specific characteristics of copper can also play a role. For example, copper ions in solution express an affinity to form coordinate bonds with available lone-pair electrons of nitrogen atoms in amines (Reaction 2) .

Reaction 2If excess amine ligands are present, coordinate bonds like that shown in Reaction 2 will continue to form until each Cu²⁺ ion in solution is coordinated with four nitrogen atoms. This acquisition of copper ions by ligands in solution can also impact the solubility equilibrium of copper salts by forming soluble coordination complexes.
-For the separation of a saturated aqueous mixture of CuF₂ (K_sp = 1.6 × 10⁻⁶) and BaF₂ (K_sp = 3.0 × 10⁻⁶) , a lab technician chose to use the common ion effect to precipitate CuF₂ from the solution rather than BaF₂. Which of the following statements does NOT explain why precipitating CuF₂ is the better choice for the separation procedure?

Quizplus · Accepted Answer

11ecba2d_120a_928b_a3bf_93ca138e08f4_MD0008_00 When an ionic compound AX₂ is placed in water, the composite ions A²⁺ and X⁻ will dissociate to some extent into the solution and establish an equilibrium in which the rate of dissolution (forward reaction) equals the rate of precipitation (reverse reaction). 11ecba2d_120a_928c_a3bf_13f6d2079150_MD0008_00 This equilibrium position is indicated by the solubility product constant K_sp, which provides a measure of the solubility of a compound at a given temperature. 11ecba2d_120a_b99d_a3bf_19e399e3d173_MD0008_00 Compounds with larger K_sp values are more soluble than compounds with smaller K_sp values. Because the K_sp of CuF₂ (1.6 × 10⁻⁶) is smaller than the K_sp of BaF₂ (3.0 × 10⁻⁶), a saturated aqueous mixture of CuF₂ and BaF₂ will have fewer moles of CuF₂ present in the solution and less CuF₂ to remove from the solution during a separation (Choice C). The K_sp of a compound is constant at a given temperature, and the concentrations of dissolved species cannot increase beyond that consistent with the K_sp. This means that if [A²⁺] increases, [X⁻] must decrease so that the value of the K_sp remains unchanged. If the K_sp is small, then small changes to [A²⁺] or [X⁻] have a much larger impact than would be the case if the K_sp were large. Therefore, CuF₂ (smaller K_sp) is more responsive than BaF₂ (larger K_sp) to small changes in common ion concentrations (Choice A), and fully precipitating BaF₂ would require a greater number of common ions to be added to the solution (Choice D). Although adding F⁻ as the common ion would cause CuF₂ to precipitate, it would also cause BaF₂ to precipitate. The fact that adding F⁻ precipitates CuF₂ does not indicate why precipitating CuF₂ is a better choice than precipitating BaF₂ (Choice B). Educational objective: The solubility of a compound is indicated by the solubility product constant K_sp, and the K_sp values of two compounds can inform which compound to selectively precipitate during the separation of a compound mixture using the common ion effect.

Passage Copper Plays a Vital Role as a Trace Mineral for for Biological