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17 Additional Aspects of Solutions to Exercises Aqueous Equilibria (c) molar solubility = [F⁻] = 3x, [La³⁺] = 0.050 M + X = (0.050 + x)(3x)³; assume X is small compared to 0.050 M. 2 X = (0.050)(27 = 1.35 = (1.48 = 5.29 X = 5 M 5.29 X 10⁻⁷ mol 195.9 LaF₃ = 1.04 10⁻⁴ = 10⁻⁴ g LaF₃ 1L 1 mol 17.57 Analyze/Plan. Given a saturated solution of in contact with undissolved consider the effect of adding CaCl₂(s). The two salts have the Ca²⁺ ion in common. Solve. As CaCl₂ is added, [Ca²⁺] increases, is exceeded, and additional CaF₂ precipitates until equilibrium is reestablished. At the new equilibrium position: (a) The amount of on the bottom of the beaker increases because the added from CaCl₂ decreases the solubility of (b) The [Ca²⁺] in solution increases because of the added CaCl₂. (c) The [F⁻] in solution decreases because precipitates upon addition of CaCl₂. The product of [Ca²⁺] and the is the same. 17.58 As KI is added, [I⁻] increases, is exceeded, and additional PbI₂ precipitates until equilibrium is reestablished. At the new equilibrium position: (a) The amount of PbI₂(s) on the bottom of the beaker increases because the added I- from KI decreases the solubility of (b) The [Pb²⁺] in solution decreases because PbI₂(s) precipitates upon addition of KI. (c) The [I⁻] in solution increases because of the added KI. The product of [Pb²⁺] and the is the same. 17.59 Analyze/Plan. We are asked to calculate the solubility of a slightly-soluble hydroxide salt at various pH values. This is a common ion problem; pH tells us not only but also [OH⁻], which is an ion common to the salt. Use pH to calculate [OH⁻], then proceed as in Sample Exercise 17.12. Solve. Mn(OH)₂(s) 1L Since is set by the pH of the solution, the solubility of Mn(OH)₂ is just [Mn²⁺]. (a) = 1.6 10⁻¹³ = 2+ [Mn²⁺] 1.0 1.6 10⁻¹³ = 16 M 16 mol 1L Mn(OH)₂ 88.95 = 1423 = 1.4 10³ g Mn(OH)₂/ 1 mol Mn(OH)₂ Check. Note that the solubility of Mn(OH)₂ in pure water is 3.6 M, and the pH of the resulting solution is 9.0. The relatively low pH of a solution buffered to pH 7.0 actually increases the solubility of Mn(OH)₂. 526