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title: "A student designs an experiment to determine the solubility product constant, \\(K_{sp}\\), of calcium sulfate, \\(\\text{CaSO}_4(s)\\). The student adds an initial mass of \\(1.500\\text{ g}\\) of pure \\(\\text{CaSO}_4(s)\\) to \\(250.0\\text{ mL}\\) of distilled water and stirs the mixture at \\(25^\\circ\\text{C}\\) until equilibrium is established. The mixture is then filtered through pre-weighed filter paper to collect the undissolved \\(\\text{CaSO}_4(s)\\).  During the filtration, the student washes the precipitate on the filter paper with several portions of room-temperature distilled water instead of a chilled solution containing a common ion. The collected solid is dried to constant mass and weighed. The student calculates the molar solubility and \\(K_{sp}\\) using the relationship: \\[ \\text{Mass of dissolved }\\text{CaSO}_4 = \\text{initial mass} – \\text{mass of dry recovered solid} \\] Which of the following correctly predicts the error in the calculated value of \\(K_{sp}\\) and provides the correct justification?"
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url: "https://nerd-notes.com/ubq/123838/"
date_modified: "2026-09-28T12:30:30+00:00"
---

# A student designs an experiment to determine the solubility product constant, \(K_{sp}\), of calcium sulfate, \(\text{CaSO}_4(s)\). The student adds an initial mass of \(1.500\text{ g}\) of pure \(\text{CaSO}_4(s)\) to \(250.0\text{ mL}\) of distilled water and stirs the mixture at \(25^\circ\text{C}\) until equilibrium is established. The mixture is then filtered through pre-weighed filter paper to collect the undissolved \(\text{CaSO}_4(s)\).

During the filtration, the student washes the precipitate on the filter paper with several portions of room-temperature distilled water instead of a chilled solution containing a common ion. The collected solid is dried to constant mass and weighed. The student calculates the molar solubility and \(K_{sp}\) using the relationship:
\[ \text{Mass of dissolved }\text{CaSO}_4 = \text{initial mass} – \text{mass of dry recovered solid} \]
Which of the following correctly predicts the error in the calculated value of \(K_{sp}\) and provides the correct justification?

A student designs an experiment to determine the solubility product constant, \(K_{sp}\), of calcium sulfate, \(\text{CaSO}_4(s)\). The student adds an initial mass of \(1.500\text{ g}\) of pure \(\text{CaSO}_4(s)\) to \(250.0\text{ mL}\) of distilled water and stirs the mixture at \(25^\circ\text{C}\) until equilibrium is established. The mixture is then filtered through pre-weighed filter paper to collect the undissolved \(\text{CaSO}_4(s)\).

During the filtration, the student washes the precipitate on the filter paper with several portions of room-temperature distilled water instead of a chilled solution containing a common ion. The collected solid is dried to constant mass and weighed. The student calculates the molar solubility and \(K_{sp}\) using the relationship:
\[ \text{Mass of dissolved }\text{CaSO}_4 = \text{initial mass} - \text{mass of dry recovered solid} \]
Which of the following correctly predicts the error in the calculated value of \(K_{sp}\) and provides the correct justification?

- **A.** The calculated \(K_{sp}\) will be too high because washing with pure distilled water increases the volume of the solution, shifting the dissolution equilibrium to the right according to Le Châtelier's principle and increasing the equilibrium concentration of dissolved ions in the filtrate.
- **B.** The calculated \(K_{sp}\) will be too high because the pure wash water contains no \(\text{Ca}^{2+}\text{(aq)}\) or \(\text{SO}_4^{2-}\text{(aq)}\) (\(Q < K_{sp}\)), causing some of the collected solid to dissolve, which decreases the mass of recovered solid and leads to an overestimation of the mass of dissolved \(\text{CaSO}_4\).
- **C.** The calculated \(K_{sp}\) will be too low because washing with pure distilled water dissolves a portion of the collected solid, resulting in a lower mass of recovered solid and leading to an underestimation of the amount of \(\text{CaSO}_4\) that dissolved.
- **D.** The calculated \(K_{sp}\) will be too low because the absence of common ions in pure distilled water reduces the ionic strength of the wash solution, causing dissolved \(\text{Ca}^{2+}\text{(aq)}\) and \(\text{SO}_4^{2-}\text{(aq)}\) to re-precipitate onto the filter paper, which increases the mass of recovered solid.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123838/*