Question

Design an experiment based on solubilities to demonstrate which of two ions, \(\mathrm{Mg}^{2+}\) or \(\mathrm{Pb}^{2+},\) is contained in an aqueous solution. Solubility information about ionic compounds is given in Tables \(R-3\) and \(R-8\) on pages 969 and 974 respectively.

Short answer

Design an experiment by preparing two test tubes labeled A and B and adding equal amounts of \(\mathrm{SO}_4^{2-}\) (e.g., sodium sulfate) solution to both. Add the unknown solution containing either \(\mathrm{Mg}^{2+}\) or \(\mathrm{Pb}^{2+}\) to each test tube and record any observable differences. If a white precipitate is observed, it indicates the presence of \(\mathrm{Pb}^{2+}\) ions (PbSO4), while no precipitate indicates the presence of \(\mathrm{Mg}^{2+}\) ions (MgSO4).

Step-by-step solution

Look up solubility rules for \(\mathrm{Mg}^{2+}\) and \(\mathrm{Pb}^{2+}\) salts

Consult the solubility tables R-3 and R-8 given on pages 969 and 974. Identify which salts of \(\mathrm{Mg}^{2+}\) and \(\mathrm{Pb}^{2+}\) are soluble and which are insoluble (or have low solubility).

Find a suitable reagent to differentiate between the ions

Select a reagent that will produce different solubility results when combined with \(\mathrm{Mg}^{2+}\) and \(\mathrm{Pb}^{2+}\). For example, \(\mathrm{SO}_4^{2-}\) can be a good choice as it forms an insoluble salt with \(\mathrm{Pb}^{2+}\) (PbSO4) and a soluble salt with \(\mathrm{Mg}^{2+}\) (MgSO4).

Design the experimental procedure

Prepare two test tubes, labeled A and B. Add equal amounts of \(\mathrm{SO}_4^{2-}\) (such as sodium sulfate, Na2SO4) solution to both test tubes. Then, add the unknown solution containing either \(\mathrm{Mg}^{2+}\) or \(\mathrm{Pb}^{2+}\) to each test tube. Record any observable differences in the test tubes (e.g. precipitation, color change, etc.).

Analyze the experiment results

Based on the observations made in Step 3, determine which ion is present in the solution. If a white precipitate is observed in one of the test tubes, it indicates the presence of \(\mathrm{Pb}^{2+}\) ions, as it forms the insoluble salt PbSO4. If no precipitate is observed, it indicates the presence of \(\mathrm{Mg}^{2+}\) ions, since MgSO4 is soluble in water.

Key concepts

Ionic Compound Solubility

Understanding the solubility of ionic compounds in water is crucial for identifying the presence of various ions in a solution. Solubility refers to the ability of a substance to dissolve in a solvent, such as water. For ionic compounds, solubility is governed by rules based on the attractions between the ions in the solid and the water molecules.

For instance, \textbf{magnesium sulfate} (\textsf{MgSO}\(_4\)) is highly soluble in water due to the strong interactions between the \textsf{Mg}\(^{2+}\) ions, the \textsf{SO}\(_4^{2-}\) ions, and the water molecules. Conversely, \textbf{lead sulfate} (\textsf{PbSO}\(_4\)) is poorly soluble because the ionic attraction within the solid \textsf{PbSO}\(_4\) latticework is stronger than the interactions with water, leading to a much lower solubility.

When designing an experiment to distinguish \textsf{Mg}\(^{2+}\) from \textsf{Pb}\(^{2+}\), one must consider these solubility rules. By adding a sulfate source to the unknown \textsf{Mg}\(^{2+}\) or \textsf{Pb}\(^{2+}\) solution, different outcomes can be observed. A clear solution suggests \textsf{Mg}\(^{2+}\) due to the formation of soluble \textsf{MgSO}\(_4\), while the formation of a precipitate suggests \textsf{Pb}\(^{2+}\), as \textsf{PbSO}\(_4\) is insoluble.

Qualitative Analysis in Chemistry

Qualitative analysis involves determining the identity of chemical substances by their properties or reactions, without focusing on exact quantities. In chemistry, qualitative analysis is key when we aim to understand what substances are present in a mixture without necessarily quantifying them.

In the experimental design we are discussing, qualitative analysis is used to distinguish between \textsf{Mg}\(^{2+}\) and \textsf{Pb}\(^{2+}\) ions. By observing whether a precipitate forms when a sulfate compound is added, we can conclude which cation is present. This observation-based approach exemplifies qualitative analysis, relying on known reactivity patterns and solubility rules. If a precipitate forms, qualitatively, it indicates the presence of \textsf{Pb}\(^{2+}\) ions; no precipitate signifies \textsf{Mg}\(^{2+}\) ions.

Good qualitative analysis also requires considering potential interference from other ions and substances in the solution. Therefore, the experiment should be designed to minimize such issues, ensuring that the reagents used react significantly only with the ions of interest, and that conditions are set to optimize the visibility of those reactions.

Experimental Design in Chemistry Education

In the realm of chemistry education, effective experimental design empowers students to understand and visualize complex chemical concepts. A well-planned chemistry experiment serves as a tangible and engaging way to learn chemical principles and apply theoretical knowledge to real-world scenarios.

When designing an experiment, it's crucial to outline the objectives clearly, choose appropriate materials and methods, and ensure safety protocols are in place. In our example, the aim is to discriminate between two possible cations in a solution. This requires the consideration of specific reagents that will yield distinguishable results for each cation based on their solubility properties.

Students are encouraged to hypothesize potential outcomes and understand the underlying chemical interactions that govern such outcomes. Post-experiment analysis is another vital aspect, where students examine the data collected to draw conclusions. Through this iterative process, experimenting facilitates a deeper comprehension of chemistry concepts and nurtures scientific thinking, critical analysis, and problem-solving skills.