Question

You are presented with a white solid and told that due to careless labeling it is not clear if the substance is barium chloride, lead chloride, or zinc chloride. When you transfer the solid to a beaker and add water, the solid dissolves to give a clear solution. Next a \(\mathrm{Na}_{2} \mathrm{SO}_{4}\left(a_{4}\right)\) solution is added and a white precipitate forms. What is the identity of the unknown white solid? [Section 4.2\(]\)

Short answer

The unknown white solid is either barium chloride or lead chloride, as both reactions with sodium sulfate produce insoluble sulfate precipitates (\(\mathrm{BaSO}_{4}\) and \(\mathrm{PbSO}_{4}\)). Zinc chloride is excluded, as its reaction with sodium sulfate produces soluble zinc sulfate (\(\mathrm{ZnSO}_{4}\)), which does not form a precipitate.

Step-by-step solution

Recall solubility rules and write the balanced chemical reaction

When sodium sulfate \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) mixes with a solution containing barium \(\mathrm{Ba}^{2+}\), lead \(\mathrm{Pb}^{2+}\), or zinc \(\mathrm{Zn}^{2+}\) ions, the following reactions are possible: 1. Reaction with Barium Chloride \(\mathrm{BaCl}_{2}\): \[ \mathrm{BaCl}_{2} + \mathrm{Na}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{BaSO}_{4} + 2 \mathrm{NaCl} \] 2. Reaction with Lead Chloride \(\mathrm{PbCl}_{2}\): \[ \mathrm{PbCl}_{2} + \mathrm{Na}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{PbSO}_{4} + 2 \mathrm{NaCl} \] 3. Reaction with Zinc Chloride \(\mathrm{ZnCl}_{2}\): \[ \mathrm{ZnCl}_{2} + \mathrm{Na}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{ZnSO}_{4} + 2 \mathrm{NaCl} \]

Analyze the products for each possible reaction

We must now analyze the products of each reaction, particularly the sulfate compounds since a white precipitate is formed when the sodium sulfate solution is added. 1. Barium Sulfate (\(\mathrm{BaSO}_{4}\)): This compound is insoluble in water and will form a white precipitate when formed in a solution. 2. Lead Sulfate (\(\mathrm{PbSO}_{4}\)): This compound is also insoluble in water and will form a white precipitate when formed in a solution. 3. Zinc Sulfate (\(\mathrm{ZnSO}_{4}\)): This compound, in contrast to the previous two, is soluble in water and does not form a precipitate.

Identify the unknown white solid

Since the problem states that a white precipitate forms when the sodium sulfate solution is added, we can conclude that the unknown white solid must be either barium chloride or lead chloride. Zinc chloride would result in a soluble sulfate product with no precipitate. Therefore, the identity of the unknown white solid is either barium chloride or lead chloride.

Key concepts

Solubility Rules

Solubility rules are guidelines used to predict whether a substance will dissolve in water. These rules help determine which compounds are soluble (dissolve in water) and which are insoluble (do not dissolve in water). Understanding solubility rules is crucial in predicting the outcomes of chemical reactions, especially when dealing with precipitation reactions.
For example, many chlorides (like barium chloride and zinc chloride) are soluble in water, meaning they dissolve to form clear solutions. However, not all sulfate compounds are soluble. Barium sulfate and lead sulfate, for instance, are exceptions. According to solubility rules,

• Most sulfate salts are soluble, but when sulfate ions combine with barium, lead, or strontium ions, they form insoluble compounds.
• This means substances like barium sulfate and lead sulfate do not dissolve in water and therefore form precipitates when these ions are present in a solution.

Knowing these details allows chemists to predict when a solid will form during a chemical reaction in an aqueous solution.

Precipitation Reactions

Precipitation reactions occur when two soluble solutions are mixed, resulting in an insoluble product that settles out of the solution as a solid, called a precipitate. These reactions are a common way to identify the presence of certain ions in a solution.
In the given problem, mixing sodium sulfate (\( \mathrm{Na}_{2}\mathrm{SO}_{4} \) ) with potential solutions containing barium, lead, or zinc ions results in precipitation reactions under specific conditions.
For instance:

• When sodium sulfate is added to a solution containing barium ions from barium chloride (\( \mathrm{BaCl}_{2} \)), barium sulfate (\( \mathrm{BaSO}_{4} \)) forms as a precipitate.
• Similarly, lead ions from lead chloride (\( \mathrm{PbCl}_{2} \)) combine with sulfate ions to form lead sulfate (\( \mathrm{PbSO}_{4} \) ) precipitate.

Both barium sulfate and lead sulfate are white precipitates, allowing one to infer the presence of barium or lead in the solution. These reactions illustrate how solubility rules aid in predicting whether a precipitate will form when different ionic solutions are mixed.

Insoluble Compounds

Insoluble compounds do not dissolve in water or form slimy precipitates when introduced to suitable conditions. This property is essential when identifying unknown substances in a chemical reaction.
Conclusively understanding which compounds are insoluble helps in practical scenarios, such as the unknown white solid problem posed. Here, the formation of a white precipitate signals the presence of either barium sulfate or lead sulfate, helping to narrow down potential identities of unknown substances.

• Barium sulfate and lead sulfate are examples of insoluble compounds that precipitate out of solution upon formation.
• In contrast, zinc sulfate is soluble, indicating no precipitate will form when zinc chloride is involved.

This process demonstrates how knowledge of insoluble compounds, coupled with solubility rules and precipitation reactions, can effectively determine the composition of unknown substances in a chemical mixture.