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346 Chapter 17 Aqueous Ionic Equilibrium 17.15 When a polyprotic acid is titrated with a strong base and if and are sufficiently different, the pH curve will have two equivalence points because the two acidic protons will be titrated sequentially. The titration of the first acidic pro- ton will be completed before the titration of the second acidic proton. 17.17 The end point is the point when the indicator changes color in an acid-base titration. The equivalence point is when stoichiometrically equivalent amounts of acid and base have reacted. With the correct indicator, the end point of the titration will occur at the equivalence point. 17.19 The solubility product constant is the equilibrium expression for a chemical equation representing the dissolution of an ionic compound. The expression of the solubility product constant of is = 17.21 In accordance with Le Châtelier's principle, the presence of a common ion in solution causes the equilibrium to shift to the left (compared to its position with pure water as the solvent), which means that less of the ionic compound dis- solves. Thus, the solubility of an ionic compound is lower in a solution containing a common ion than in pure water. The exact value of the solubility can be calculated by working an equilibrium problem in which the concentration of the common ion is accounted for in the initial conditions. The molar solubility of a compound, in a solution with an initial concentration of = and an initial concentration of = can be computed directly from by solving for S in the expression = + 17.23 Q is the reaction quotient, the product of the concentrations of the ionic components raised to their stoichiometric coef- ficients, and K is the product of the concentrations of the ionic components raised to their stoichiometric coefficients at equilibrium. A solution contains an ionic compound: if Q the solution is supersaturated and under most circumstances, the excess solid will precipitate out. 17.25 Qualitative analysis is a systematic way to determine the metal ions present in an unknown solution by the selective precipitation of the ions. The word qualitative means "involving quality or kind." So qualitative analysis involves finding the kind of ions present in the solution. Quantitative analysis is concerned with quantity, or the amounts of substances in a solution or mixture. Problems by Topic The Common Ion Effect and Buffers 17.27 The only solution in which HNO₂ will ionize less is (d) 0.10 M NaNO₂. It is the only solution that generates a common ion with nitrous acid. 17.29 (a) Given: 0.20 M and 0.15 M NaCHO₂ Find: pH Other: (HCHO₂) = 1.8 10⁻⁴ Conceptual Plan: M M then M M pH NaCHO₂(aq) + ICE table pH = Solution: Because 1 ion is generated for each = 0.15 + 1L + [HCHO₂] [H₃O⁺] Initial 0.20 =0.00 0.15 Change -x +x +x Equil 0.20 - x +x = [HCHO₂] = 10⁻⁴ = x(0.15 0.20 Assume that x is small (x