A solution is prepared by dissolving of Zn(NO ) in enough water to make of solution at

CHEMISTRY · Multi-part · AP · Chemistry · Equilibrium · Equilibrium

Problem

A solution is prepared by dissolving 0.0200 mol0.0200\,\text{mol} of Zn(NO3_3)2_2 in enough water to make 500.0 mL500.0\,\text{mL} of solution at 25 ∘C25\,^{\circ}\text{C}. Solid NaOH is slowly added with constant stirring until the moment Zn(OH)2_2 begins to appear. Data at 25 ∘C25\,^{\circ}\text{C}: Zn(OH)2 (s)⟷Zn2+ (aq)+2 OH−;Ksp=3.0×10−10.\text{Zn(OH)}_2\,(s) \longleftrightarrow \text{Zn}^{2+}\,(aq) + 2\,\text{OH}^{-};\quad K_{sp} = 3.0 \times 10^{-10}. (Assume activity coefficients are unity.) (a) Determine the concentration of Zn2+^{2+} in the solution just before any NaOH is added. (b) Calculate the hydroxide-ion concentration required for the first traces of Zn(OH)2_2 to form. (c) Using your result from part (b), compute the mass of solid NaOH (molar mass 40.00 g/mol40.00\,\text{g/mol}) that must be added to reach this point, assuming the final volume change is negligible. (d) Explain qualitatively how the answer to part (c) would change if the dissolution were carried out in 0.10 mol L−10.10\,\text{mol}\,\text{L}^{-1} NH3_3(aq) instead of pure water.

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