Problem 32 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following sulphate is insoluble in water? (a) \(\mathrm{CdSO}_{4}\) (b) \(\mathrm{PbSO}_{4}\) (c) \(\mathrm{CuSO}_{4}\) (d) \(\mathrm{Bi}_{2}\left(\mathrm{SO}_{4}\right)_{3}\)

Step-by-Step Solution

Answer

\( \mathrm{PbSO}_4 \) is the insoluble sulfate.

Step 1: Understand Solubility Rules

To solve this problem, we need to know the solubility rules for sulfates. Generally, most sulfates are soluble in water, but there are exceptions. The common exceptions include Barium Sulfate (\( \mathrm{BaSO}_4 \)), Lead Sulfate (\( \mathrm{PbSO}_4 \)), and Calcium Sulfate (\( \mathrm{CaSO}_4 \)), which are largely insoluble.

Step 2: Identify the Given Compounds

List the given sulfate compounds in the problem: (a) \( \mathrm{CdSO}_4 \), (b) \( \mathrm{PbSO}_4 \), (c) \( \mathrm{CuSO}_4 \), and (d) \( \mathrm{Bi}_2(\mathrm{SO}_4)_3 \).

Step 3: Apply Solubility Rules

Using our understanding from Step 1, identify which of these sulfates is an exception to the solubility rules. Here, \( \mathrm{PbSO}_4 \) is one of the known exceptions and is insoluble in water.

Step 4: Compare with Other Compounds

Quickly analyze the other compounds to confirm their solubility: \( \mathrm{CdSO}_4 \), \( \mathrm{CuSO}_4 \), and \( \mathrm{Bi}_2(\mathrm{SO}_4)_3 \) are generally soluble in water according to the solubility rules.

Step 5: Choose the Correct Option

Since \( \mathrm{PbSO}_4 \) is identified as insoluble in water, option (b) is the correct choice.

Key Concepts

Sulfate Compounds

Sulfate compounds are a class of chemical compounds that include the sulfate ion, denoted as \(\mathrm{SO}_4^{2-}\). These compounds are commonly found in nature and have a wide range of industrial and chemical applications.
Sulfates are typically formed when metals react with sulfuric acid. The resulting compound's solubility in water greatly depends on the metal involved. Some metals form highly soluble sulfate compounds, while others form precipitates when combined with the sulfate ion.

• Metal sulfates like sodium sulfate \(\mathrm{Na}_2\mathrm{SO}_4\) and potassium sulfate \(\mathrm{K}_2\mathrm{SO}_4\) are known for their high water solubility.
• Sulfates of more complex or heavy metals can behave differently, showing varying levels of solubility.

To understand if a particular sulfate is soluble, it is essential to consider the metal it is paired with since this determines the compound's behavior in aqueous solutions.

Insoluble Sulfates

While many sulfates dissolve well in water, there are notable exceptions known as insoluble sulfates. These sulfates are resistant to dissolving and typically form a solid precipitate when introduced to an aqueous environment. Insoluble sulfates include some of the heavier metals:

• Barium sulfate \(\mathrm{BaSO}_4\)
• Lead sulfate \(\mathrm{PbSO}_4\)
• Calcium sulfate \(\mathrm{CaSO}_4\)

Lead sulfate, \(\mathrm{PbSO}_4\), is especially known for its low solubility, making it a common example when discussing solubility exceptions in sulfates. This pattern of solubility is crucial in various scientific and industrial processes, such as the treatment of wastewater or in the production of certain pigments.

Chemical Solubility Exceptions

In chemical solubility, exceptions often arise that defy the general rules, and understanding these exceptions is vital for correct predictions and applications.
The general rule for sulfates is that they are water-soluble; however, exceptions like lead sulfate \(\mathrm{PbSO}_4\) are significant because they do not dissolve in water. This occurs due to the specific interactions between the ions in the solid state and the surrounding water molecules.

• Exceptions to solubility rules can often be attributed to an imbalance in intermolecular forces and lattice energy.
• Metals in insoluble sulfates typically have a high charge and small radius, leading to strong ionic bonds in the solid.
• Such bonds are not easily broken by solvation, which is why these compounds do not dissolve readily.

Recognizing these exceptions helps in predicting reactions in both laboratory and natural settings, aiding in the decision-making process in chemistry.