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

Option (a) is correct: 1000 g of the solvent.

Step 1: Understanding the Definition of Molality

Molality is a measure of the concentration of a solute in a solution, defined as the number of moles of solute per kilogram of solvent. Therefore, a 1 molal solution contains 1 mole of solute in 1 kg (or 1000 g) of solvent.

Step 2: Identifying the Correct Option

Review the given options to determine which one matches the definition of molality: - Option (a) specifies 1000 g of solvent, which aligns with the definition. - Options (b) and (c) involve volume measurements, which relate to molarity, not molality. - Option (d) contains a volume at standard conditions, which is not relevant to molality.

Step 3: Conclusion on the Correct Answer

Since molality focuses on mass, not volume, the correct answer to match the definition of a molal solution is option (a), which specifies 1000 g of the solvent.

Key Concepts

Concentration Measurements

Measuring concentration is crucial in chemistry, as it's essential to know how much of a solute is present in a solution. Different methods exist to express concentration, but the most common are molarity and molality.

• Molarity (M): This measures the number of moles of solute per liter of solution. It depends on the volume of the entire solution, which can change with temperature.
• Molality (m): This indicates the number of moles of solute per kilogram of solvent. Unlike molarity, molality is based on mass, and therefore, it is not affected by temperature changes.

When working with solutions, choosing the correct measure of concentration is important depending on the conditions of your experiment or application. Molality is particularly useful in scenarios involving temperature changes.

Solution Chemistry

Solution chemistry explores how substances dissolve, interact, and form mixtures at the molecular level. Understandably, solutions are ubiquitous in both nature and industry.
A solution is made by dissolving a solute in a solvent. Solvent is typically more abundant and serves as the medium in which the solute is dispersed. When both are fully mixed, a homogeneous solution is formed.
In solution chemistry, key properties such as solubility, concentration, and the effects of temperature and pressure on the solution are studied. These factors influence how much solute can be dissolved and affect the physical and chemical properties of the resulting mixture.
Ultimately, understanding solution chemistry is vital for applications ranging from pharmaceuticals to everyday household products.

Moles and Mass Relation

To grasp how substances interact in chemistry, it's important to understand the concept of moles and mass.

• Mole: A mole is a unit representing a quantity of particles, usually atoms or molecules. Specifically, one mole contains approximately 6.022 x 10²³ particles (Avogadro's number).
• Mass: In chemistry, mass relates to the weight of substances and how they react. Each element has an atomic mass, which is typically listed on the periodic table.

By calculating the moles and using the substance's molar mass (the mass of one mole of that substance), chemists can measure and predict the outcomes of reactions with precision. This relationship is foundational in creating solutions with known concentrations, like the molal solutions detailed in the exercise. Molality is a perfect demonstration of integrating these two concepts seamlessly by relating moles of solute to the mass of the solvent.