A geochemical study investigates how the stability of several sparingly soluble…
CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
A geochemical study investigates how the stability of several sparingly soluble minerals is influenced by the acidity of groundwater. Three powdered solids—barium sulfate (BaSO4), lead(II) chloride (PbCl2), and zinc sulfide (ZnS)—are mixed in equal masses and placed in beakers containing 1.0L of aqueous solution under the temperature conditions specified by the following solubility-product constants.
Ksp[BaSO4] = 1.1×10−10
Ksp[PbCl2] = 1.7×10−5
Ksp[ZnS] = 2.5×10−22
You may assume that temperature is constant and that no complex ion formation occurs other than simple protonation or deprotonation of the relevant anions.
(a) For each anion (SO42−, Cl−, and S2−), state whether it can act as a Brønsted–Lowry base under acidic conditions. Support each answer with a concise explanation.
(b) The pH of the groundwater sample is abruptly lowered to 3.0. Identify which of the three minerals will exhibit a significant increase in solubility at this pH relative to neutral pH. Justify each choice in terms of chemical equilibria.
(c) The pH is then slowly raised from 3.0 to 11.0. For each mineral, predict whether its solubility will increase, decrease, or remain approximately constant over this pH range. Provide chemical reasoning for each prediction.
(d) Rank the three minerals in order of increasing overall sensitivity to pH changes between 3.0 and 11.0, and explain the basis for your ranking.