Problem 18 - A Complete Resource Book in Chemistry for JEE Main

Question

Highest boiling point is found in (a) \(0.1 \mathrm{M}\) Sucrose (b) \(0.1 \mathrm{M} \mathrm{BaCl}_{2}\) (c) \(0.1 \mathrm{M} \mathrm{NaCl}\) (d) \(0.1 \mathrm{M} \mathrm{KCl}\)

Step-by-Step Solution

Answer

Highest boiling point is in 0.1 M BaCl₂.

Step 1: Understand boiling point elevation

The boiling point of a solution increases when a solute is dissolved in a solvent. The elevation in boiling point depends on the number of solute particles in the solution. This is known as boiling point elevation.

Step 2: Identify the dissociation of compounds

Next, we identify how each substance dissociates in solution. Sucrose ( ext{C}_{12} ext{H}_{22} ext{O}_{11}) does not dissociate: it remains as 1 particle. Barium chloride ( ext{BaCl}_2) dissociates into 3 ions ( ext{Ba}^{2+} and 2 ext{Cl}^{-}). Sodium chloride ( ext{NaCl}) dissociates into 2 ions ( ext{Na}^{+} and ext{Cl}^{-}). Potassium chloride ( ext{KCl}) also dissociates into 2 ions ( ext{K}^{+} and ext{Cl}^{-}).

Step 3: Calculate the boiling point elevation based on ions

The boiling point elevation is proportional to the number of ions produced. For 0.1 M solutions, the effective concentration regarding ion count is as follows: Sucrose = 0.1 M, BaCl_2 = 0.3 M (0.1 M * 3 ions), NaCl = 0.2 M (0.1 M * 2 ions), KCl = 0.2 M (0.1 M * 2 ions).

Step 4: Compare boiling point elevations

Since boiling point elevation is highest with the greatest number of particles, the solution with BaCl_2, which yields the most ions (0.3 M in effective ion concentration), will have the highest boiling point elevation.

Key Concepts

Dissociation of Compounds

When a compound is dissolved in a solvent, it may dissociate, or split, into smaller particles. This process is particularly relevant for ionic compounds. Ionic compounds like sodium chloride (aCl]) break apart into their constituent ions when dissolved in water. For instance, NaCl dissociates into

• sodium ions (a^{+}])
• chloride ions (l^{-}])

Potassium chloride (Cl]) behaves similarly, and when barium chloride (aCl_{2}]) dissolves, it forms three ions:

• one barium ion (a^{2+}]), and
• two chloride ions (l^{-}])

However, substances like sucrose do not dissociate. Sucrose stays as one whole molecule in the solution, which means it doesn't increase the number of solute particles.

Solute Particles

The presence of solute particles in a solution is crucial for determining particular properties of the solution, such as boiling point elevation. Solute particles are the dissolved ions or molecules from a compound, and their number results from the dissociation process. When a solute is dissolved:

• The more it dissociates, the more particles it creates.
• The more particles that are created, the greater the impact on properties like boiling point and freezing point.

For example, in a 0.1 M solution:

• Sucrose provides 0.1 M particles because it does not dissociate.
• aCl] forms 0.2 M of ions due to each molecule producing two ions.
• aCl_{2}] results in 0.3 M of ions, yielding the most particles due to its break into three ions.

The solution with the highest concentration of solute particles tends to demonstrate the most significant boiling point elevation.

Solution Concentration

Solution concentration refers to the amount of solute that is dissolved in a given quantity of solvent. Concentration, often expressed in molarity (M), indicates how crowded the solute particles are within the solution.
A higher concentration means more solute particles per unit of solvent, which can lead to more noticeable changes in properties such as boiling or freezing points. In our case of boiling point elevation:

• Sucrose, with a concentration of 0.1 M, remains at this concentration because it does not produce additional particles.
• aCl] increases its effective concentration due to ion dissociation, reaching 0.2 M.
• aCl_{2}] has the highest effective concentration of 0.3 M because of its three dissociated ions.

Thus, solution concentration directly influences the degree of boiling point elevation, with higher effective concentrations leading to more significant elevations.