Problem 8 - Chemistry: The Central Science

Question

The labels have fallen off two bottles, one containing \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\) and the other containing \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\). You have a bottle of dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}\). How could you use it to test a portion of each solution to identify which solution is which? [Section 4.2]

Step-by-Step Solution

Answer

To identify which solution is Mg(NO3)2 and which is Pb(NO3)2 using dilute H2SO4, take small samples from each unknown solution and add H2SO4 to each. Observe the reactions: - If a white precipitate forms, the solution is Pb(NO3)2 (due to the formation of PbSO4). - If the solution remains clear, the solution is Mg(NO3)2 (due to the formation of soluble MgSO4).

Step 1: Write down the chemical reactions

When Mg(NO3)2 or Pb(NO3)2 reacts with H2SO4, the following reactions take place: \[Mg(NO_3)_2 + H_2SO_4 \rightarrow MgSO_4 + 2HNO_3\] and \[Pb(NO_3)_2 + H_2SO_4 \rightarrow PbSO_4 + 2HNO_3\]

Step 2: Determine the difference in the reaction products

The products of the reactions are MgSO4 and PbSO4. The key difference between MgSO4 and PbSO4 is their solubility in water. MgSO4 is soluble in water and forms a clear solution, while PbSO4 is not very soluble and forms a white precipitate.

Step 3: Perform the test with dilute H2SO4

To identify which solution is which, follow these steps: 1. Take two small samples, one from each of the two unknown solutions. 2. Add a small amount of dilute H2SO4 to each sample. 3. Observe the reactions for any formation of precipitate.

Step 4: Analyze the results

After adding dilute H2SO4: - If a white precipitate is formed, the solution is Pb(NO3)2 (due to the formation of PbSO4). - If the solution remains clear, the solution is Mg(NO3)2 (due to the formation of soluble MgSO4). This way, you can identify which solution contains Mg(NO3)2 and which contains Pb(NO3)2.

Key Concepts

Solubility

Solubility is a critical property of substances, especially when discussing chemical reactions involving solutions. It refers to a substance's ability to dissolve in a solvent, like water. Solubility is influenced by factors such as temperature, pressure, and the nature of both the solute and solvent.

In the context of the given exercise, understanding solubility helps differentiate between the two products formed during the reactions.

• Magnesium sulfate ( MgSO_4 ) is highly soluble in water. As a result, when it forms during the reaction, it readily dissolves, leaving the solution clear.
• Lead sulfate ( PbSO_4 ), on the other hand, has low solubility in water. This means it does not dissolve easily, leading to the formation of a precipitate.

The differing solubility of MgSO_4 and PbSO_4 is the key to identifying the unknown solutions.

Precipitate Formation

Precipitation occurs when a solution reacts to form a solid, known as a precipitate. This is a crucial phenomenon in chemistry because it often indicates the formation of a new compound.

In this exercise, Pb(NO_3)_2 reacts with H_2SO_4 to form PbSO_4 , which is insoluble in water. As a result:

• PbSO_4 appears as a white solid precipitate in the reaction mixture.
• This solid formation suggests that the reaction mixture contains lead.

Being able to visually detect a precipitate allows chemists to identify substances present in a solution. The absence or presence of a precipitate is a simple yet powerful tool in chemical analysis.

Solution Identification

Identifying solutions based on their chemical properties, such as solubility and precipitate formation, is a practical application of chemistry.

In the scenario given, using H_2SO_4 to distinguish between Mg(NO_3)_2 and Pb(NO_3)_2 depends on observing the reaction's outcome:

• If a white precipitate forms, the solution was Pb(NO_3)_2 , due to the creation of PbSO_4 , which is insoluble.
• If no precipitate forms and the solution remains clear, the solution was Mg(NO_3)_2 because MgSO_4 is soluble.

This method showcases how chemical properties can be employed for solution identification without advanced equipment, relying solely on simple tests and visual observations.