Problem 132 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following sulphates are soluble in water? (a) \(\mathrm{PbSO}_{4}\) (b) \(\mathrm{CuSO}_{4}\) (c) \(\mathrm{BaSO}_{4}\) (D) \(\mathrm{Ag}_{2} \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

Copper sulfate (CuSO₄) is soluble in water.

Step 1: Understand Solubility Rules

In chemistry, solubility rules help predict which ionic compounds are soluble or insoluble in water. According to these rules, most sulfate salts are soluble in water, with exceptions including sulfates of lead (Pb), barium (Ba), calcium (Ca), and silver (Ag) due to lower solubility.

Step 2: Analyze Compound (a) PbSO4

Lead sulfate (\( \mathrm{PbSO}_{4}\)) is an exception to the sulfate solubility rule, making it insoluble in water. This means that \( \mathrm{PbSO}_{4}\) does not dissolve well in water.

Step 3: Analyze Compound (b) CuSO4

Copper sulfate (\( \mathrm{CuSO}_{4}\)) generally dissolves well in water, adhering to the sulfate solubility rule. Thus, \( \mathrm{CuSO}_{4}\) is soluble in water.

Step 4: Analyze Compound (c) BaSO4

Barium sulfate (\( \mathrm{BaSO}_{4}\)) is another exception to the sulfate solubility rule, making it insoluble in water. Thus, \( \mathrm{BaSO}_{4}\) does not dissolve well in water.

Step 5: Analyze Compound (d) Ag2SO4

Silver sulfate (\( \mathrm{Ag}_{2} \mathrm{SO}_{4}\)) is somewhat soluble in water but much less so compared to other sulfates. It is often considered sparingly soluble, forming a cloudy solution due to some undissolved particles.

Step 6: Conclude Solubility Analysis

Based on the analysis, \( \mathrm{CuSO}_{4}\) is soluble, while \( \mathrm{PbSO}_{4}\), \( \mathrm{BaSO}_{4}\), and \( \mathrm{Ag}_{2} \mathrm{SO}_{4}\) are not considered soluble in water.

Key Concepts

Sulfates

Sulfates are salts that contain the sulfate ion, \( \mathrm{SO}_{4}^{2-} \), and are commonly encountered in chemistry classes. Many sulfate salts are soluble in water, making them an important class of compounds in various reactions and applications. When a compound is soluble, it means that it can dissolve in water, breaking down into its constituent ions.

However, not all sulfates behave this way. Most sulfate salts, often including sodium sulfate and potassium sulfate, dissolve readily in water. This characteristic solubility makes them useful in various industrial and laboratory processes. Understanding which sulfates are soluble is crucial for predicting reaction outcomes and for industrial applications. Recognizing sulfates that do not dissolve well involves knowledge of certain exceptions, which we'll explore in detail.

Insoluble Compounds

Insoluble compounds are substances that do not dissolve significantly in water, remaining mostly in solid form. These compounds can form precipitates, which are solid particles that settle out of a solution.

In the context of sulfates, some specific compounds are known to be insoluble:

• Lead sulfate \( \mathrm{PbSO}_{4} \)
• Barium sulfate \( \mathrm{BaSO}_{4} \)
• Calcium sulfate \( \mathrm{CaSO}_{4} \)
• Silver sulfate \( \mathrm{Ag}_{2} \mathrm{SO}_{4} \)

These compounds are notable exceptions to the usual behavior of sulfate solubility, leading to their classification as insoluble or, in some cases, sparingly soluble. The presence of such sulfates in a reaction can lead to the formation of a solid product if mixed with soluble solutions containing appropriate ions. Students should remember that not all salts behave the same; some are more likely to remain undissolved based on their unique properties.

Solubility Exception

The concept of solubility exceptions is a common theme in chemistry, especially useful when predicting the behavior of ionic compounds in aqueous solutions. Certain ions cause specific compounds to defy the general solubility rules, for instance, causing commonly soluble ions like sulfates to become insoluble.

Key to understanding these exceptions is recognizing the role of ionic charge and lattice energy. When a sulfate combines with ions like lead (Pb), barium (Ba), calcium (Ca), or silver (Ag), the strong ionic bonds in these compounds prevent them from easily breaking apart in water.

• Lead, barium, and calcium form strong ionic lattices with sulfates, resisting dissolution.
• Silver sulfate, due to its unique silver ions, dissolves less readily than other sulfates.

Knowing these solubility exceptions helps chemists determine whether a precipitate will form in a reaction. Students often encounter these exceptions as practical examples where typical predictions encounter unique behavior, establishing a deeper understanding of chemical interactions.