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 (a).

Step 1: Understanding Molality

Molality is defined as the concentration of a solution expressed as the number of moles of solute per kilogram of solvent. It is measured in moles per kilogram.

Step 2: Identify Given Options

We have the following options: (a) 1000 g of solvent, (b) one litre of solvent, (c) one litre of solution, and (d) 22.4 litres of the solution. We need to choose the option that matches the definition of molality.

Step 3: Evaluate Each Option

(a) 1000 g of solvent is equivalent to 1 kg of the solvent, matching the molality definition. (b) One litre of solvent does not match the mass requirement needed for molality. (c) One litre of solution considers the total volume, not solvent mass. (d) 22.4 litres of solution also considers total volume, not mass of solvent.

Step 4: Select the Correct Answer

Based on the evaluation, option (a) matches the definition of a molal solution: one mole of solute in 1000 g (1 kg) of solvent.

Key Concepts

Molal Solution

Molal solutions are an essential concept in chemistry, helping us understand the concentration of solute in a solution related to the mass of the solvent. Unlike molarity, which considers the volume of the entire solution, molality focuses on the mass of the solvent alone. This distinction is crucial because physical properties like temperature and pressure can affect the volume but not the mass.

• A molal solution contains precisely one mole of solute.
• It requires exactly 1000 grams (or 1 kilogram) of solvent.
One key advantage of using molality is its independence from temperature changes, making it very reliable in environments where temperature can fluctuate.
If you were to prepare a molal solution, you would measure out one mole of your substance—say, sugar—and dissolve it in exactly 1 kg of water, ensuring the solution remains consistent across different conditions.

Concentration of Solution

The concentration of a solution tells us how much solute is present in a given quantity of solvent or solution. It is a way to quantify the amount of solute dispersed in the solvent and can be expressed in various units depending on the context.
For molal solutions, concentration is specifically represented in moles per kilogram of solvent, distinguishing it from other measures like molarity, which uses liters of solution.

• In molality, the formula is: \( ext{molality} = \frac{ ext{moles of solute}}{ ext{kilograms of solvent}} \).
• It requires knowing the precise weight of the solvent for an accurate calculation.

By focusing on the mass of the solvent, molality offers a precise measurement that remains unaffected by conditions affecting volume, such as pressure or temperature changes.

Moles of Solute per Kilogram of Solvent

In chemistry, the amount of substance is often measured in moles. A mole is a specific quantity—about \( 6.022 \times 10^{23} \)—of molecules or atoms. When it comes to solutions, especially molal solutions, understanding this concept is crucial.
Molality calculates the concentration based on the moles of solute per kilogram of solvent. This ratio is both simple and practical for many chemical calculations and reactions.

• You need to accurately measure the number of moles of solute.
• The weight of solvent, precisely measured in kilograms, is equally important.
• This ratio aids in comparing concentration levels without considering the solution's overall volume.

Ensuring you know the exact number of solute moles and the corresponding mass of the solvent is essential to correctly determining the molality, making it a valuable tool in analytical and physical chemistry.