Problem 33 - A Complete Resource Book in Chemistry for JEE Main

Question

A molal solution is one that contains one mole of a solute in (a) \(1000 \mathrm{~g}\) of the solvent (b) one litre of solvent (c) one litre of solution (d) \(22.4\) litre of the solution

Step-by-Step Solution

Answer

A molal solution contains one mole of solute in 1000 g of solvent (Option a).

Step 1: Understand Molality

Molality is a way to express the concentration of a solution. It is defined as the number of moles of solute per 1000 grams (or kilograms) of solvent. Molality (m) focuses solely on the weight of the solvent, not the entire solution.

Step 2: Apply the Definition

By the definition of molality, it is clear that a molal solution requires one mole of solute to be dissolved in exactly 1000 grams of the solvent.

Step 3: Evaluate Options

Now, evaluate each option against the definition: - Option (a) states one mole of solute in 1000 grams of solvent, which exactly matches the definition of molality. - Option (b) involves one litre of the solvent, which is about volume, not weight, hence not applicable for molality. - Option (c) and (d) also involve the volume of the solution rather than the mass of the solvent, making them incorrect as well for molality.

Step 4: Select the Correct Answer

Through the evaluation, option (a) is the only one that matches the criteria of a molal solution, where one mole of solute is dissolved in 1000 grams of solvent.

Key Concepts

Concentration

Concentration is a fundamental concept used to describe how much solute is present in a given amount of solvent or solution. It reflects the strength of the solution and is crucial for calculations in chemistry.

Concentration can be expressed in different ways, such as molarity, molality, and normality. Each method highlights a specific aspect of the solution, such as volume or mass. Molality, for instance, measures concentration based on mass.

In the context of molality, concentration is demonstrated by the moles of solute per 1000 grams of solvent. Unlike molarity, which depends on the solution's volume, molality is based solely on the mass of the solvent. This makes it an ideal measurement for situations where temperature and pressure may vary, as it remains consistent regardless of these changes.

Mole of Solute

A mole is a basic unit in chemistry that represents a specific number of particles, usually atoms or molecules, typically described as Avogadro's number, approximately equal to \(6.022 \times 10^{23}\).

When discussing the mole of a solute, we are referring to the amount of the substance being dissolved in the solvent. For molal calculations, one mole of the solute is the standard measure to express the concentration.

• 1 mole of a solute in a molal solution specifically refers to that quantity being dissolved in enough solvent to make 1000 grams, not concerning the overall solution volume.
• This measure allows chemists to perform precise calculations in reaction predictions and scaling, critical in both laboratory and industrial applications.

Using the mole as a unit cultivates a universal understanding in chemical reactions, enabling consistency in expressing concentration and performing accurate stoichiometric calculations.

Mass of Solvent

Understanding the mass of the solvent is key to calculating molality. Solvent mass is measured in grams and directly influences the solute's concentration level within a solution.

For a molal solution, exactly 1000 grams (or 1 kilogram) of the solvent is required to dissolve one mole of solute. This consistency ensures that molality remains unaffected by temperature changes, as it does not rely on volume measurements like molarity, which can fluctuate with temperature.

The importance of solvent mass is clearly observed in practical applications:

• It enables chemists to prepare solutions with precise concentrations needed for specific reactions.
• Focus on the solvent's mass rather than volume aids in accurate measurements in varying environmental conditions.

Thus, mastering the concept of solvent mass is important for achieving desired solution concentrations and maintaining experimental fidelity.