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

Option (b) \( \mathrm{PbSO}_{4} \) is insoluble in water.

Step 1: Reviewing Sulfate Solubility Rules

Generally, most sulfates are soluble in water. However, some exceptions exist, such as sulfates of lead (Pb), barium (Ba), and calcium (Ca), which are insoluble.

Step 2: Analyze the Options

Analyze the provided options to check which sulfates are exceptions to the general rule of solubility. - (a) \( \mathrm{CdSO}_{4} \) : Cadmium sulfate is generally soluble in water.- (b) \( \mathrm{PbSO}_{4} \) : Lead sulfate is an exception and is insoluble in water.- (c) \( \mathrm{CuSO}_{4} \) : Copper sulfate is soluble in water.- (d) \( \mathrm{Bi}_{2}\left(\mathrm{SO}_{4}\right)_{3} \) : Bismuth sulfate is generally soluble in water.

Step 3: Choose the Correct Answer

From the analysis in Step 2, we find that only lead sulfate (\( \mathrm{PbSO}_{4} \) ) is insoluble in water. Therefore, the correct answer is (b) \( \mathrm{PbSO}_{4} \).

Key Concepts

Insoluble Sulfates

Sulfates are compounds consisting of sulfur and oxygen, combined with various metals or other elements. While many sulfates dissolve in water, there are specific sulfates that are insoluble. These insoluble sulfates do not readily dissolve in water because they form lattice structures that are highly stable and tightly bonded. It means that water molecules can't easily separate the ions of the sulfate from one another.
Common examples of insoluble sulfates include:

• Lead sulfate (PbSO₄)
• Barium sulfate (BaSO₄)
• Calcium sulfate (CaSO₄)

These compounds do not dissolve easily in water, making them exceptions to the usual behavior of sulfates, which tend to be soluble. The presence of an insoluble sulfate can lead to the formation of precipitates in chemical reactions, which are solid substances that form in a solution and settle out.

Solubility Rules

Solubility rules are guidelines that help predict whether a compound will dissolve in water. They are essential principles in chemistry, providing insight into why some substances dissolve while others do not.
These rules are particularly useful in identifying exceptions, as seen with sulfates. While most sulfate compounds dissolve in water, solubility rules highlight important exceptions, such as lead sulfate, barium sulfate, and calcium sulfate.
For sulfates, the general solubility rule states:

• "Most sulfate salts are soluble in water except for those of calcium, barium, and lead."

These rules help chemists understand and predict outcomes in reactions, particularly when mixing different solutions. Knowing these exceptions prevents mistakes in laboratory settings, where assuming a sulfate compound is soluble could lead to unexpected precipitation.

Lead Sulfate

Lead sulfate ( PbSO⁴) is a classic example of an insoluble sulfate. Its insolubility is mainly due to the strong ionic bonds between the lead ions (Pb²⁺) and sulfate ions (SO₄²⁻), forming a robust lattice structure that does not easily break apart in water.
When you mix solutions containing lead ions and sulfate ions, lead sulfate precipitates out as a solid because it does not dissolve. This characteristic is crucial in various applications and environments:

• In water treatment processes, where ensuring the removal of lead ions is essential.
• In laboratory experiments and industrial processes, predicting precipitate formation is necessary for accurate results.
• In the natural environment, the formation of lead sulfate can affect soil and water quality.

Understanding the chemistry of lead sulfate, and related compounds, is vital to anticipate their behavior in different chemical contexts.