CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
A sparingly soluble divalent metal hydroxide, M(OH)2, is in equilibrium with pure water at 25∘C.
Solubility-product data: Ksp=2.0×10−10.
A student investigates how the extent of precipitation of M(OH)2 changes as the pH of the solution is lowered by adding a strong monoprotic acid.
Answer the following questions. (Assume temperature remains constant and activity coefficients are unity.)
(a) Write the balanced chemical equation for the dissolution of solid M(OH)2 in water and the corresponding symbolic expression that relates the concentrations of the dissolved ions at equilibrium.
(b) Describe, without performing any calculations, how the concentration of dissolved M2+ ions will change when the pH is decreased from 10.00 to 7.00. Your explanation should refer explicitly to changes in ionic species present and shifts in equilibrium.
(c) Explain why a sufficiently low pH ultimately prevents any precipitation of M(OH)2, even if excess solid is present. Include a discussion of the relative magnitudes of the ionic product and the solubility product under acidic conditions.