Problem 45 - A Complete Resource Book in Chemistry for JEE Main

Question

Camphor is used as a solvent to determine molecular weight of non-volatile solute by Rast method because for camphor (a) its molal depression constant is high (b) being very cheap (c) high melting point (d) all of these

Step-by-Step Solution

Answer

(d) all of these

Step 1: Understanding the Question

The question asks why camphor is used as a solvent to determine the molecular weight of a non-volatile solute using the Rast method. Several options are given, and the correct one needs to be identified.

Step 2: Evaluating Each Option

We need to evaluate each option one by one to see which factors make camphor suitable for the Rast method. - (a) Its molal depression constant is high: This means camphor will cause a large depression in freezing point for small amounts of solute, making it easier to measure precise changes. - (b) Being very cheap: Cost-effectiveness is always a valid reason, but not the most scientific one. - (c) High melting point: A high melting point means it's easier to handle without the solvent evaporating at room temperature. - (d) All of these: This option suggests that camphor could be selected due not just to a single reason, but a combination of all these factors.

Step 3: Selecting the Most Comprehensive Option

Camphor is often used for its significant molal depression constant, making it a practical choice for calculating small differences in molecular weight accurately. However, both cost-effectiveness and a high melting point contribute to its practical usability. Thus, option (d) is the most comprehensive as it includes all relevant factors that make camphor suitable for the Rast method.

Key Concepts

Molal Depression Constant

The molal depression constant, also known as the cryoscopic constant, is a property of a solvent that measures how much its freezing point decreases when a solute is added. This is crucial in certain analytical chemistry methods, like the Rast method, where it helps in determining the molecular weight of solutes. A higher molal depression constant implies a greater freezing point depression for every mole of solute dissolved. This makes it easier to detect even small changes in the freezing point, which translates to more accurate measurements. For substances like camphor, which have a high molal depression constant, it facilitates precision in experiments involving molecular weight determination.

Molecular Weight Determination

Determining molecular weight is essential in chemistry to understand a compound’s relative size and mass. The Rast method, a well-known technique for this purpose, relies on the freezing point depression principle. When a solute is dissolved in a solvent like camphor, the solution experiences a lowering of its freezing point. By measuring this decrease and knowing the solvent's molal depression constant, scientists can calculate the molecular weight of the solute using the formula: \[\text{Molecular weight of solute} = \left( \frac{K_f \times W_s}{\Delta T_f \times W_{sol}} \right) \]Where \(K_f\) is the molal depression constant, \(W_s\) is the weight of the solvent, \(\Delta T_f\) is the change in freezing point, and \(W_{sol}\) is the weight of the solute. This method is particularly useful for non-electrolyte solutes.

Non-Volatile Solute

In the context of the Rast method, a non-volatile solute is a compound that does not readily evaporate. This is important because if a solute were volatile, it might evaporate during the experiment, leading to inaccurate measurements. Non-volatile solutes ensure that the entirety of the solute remains within the solution, providing consistent results when determining molecular weight. Their stability during the experiment helps maintain the integrity of the data collected for the freezing point depression, which is critical for precise calculations.

Camphor Solvent Properties

Camphor is an organic compound with several characteristics that make it advantageous as a solvent in freezing point depression methods.

• High Molal Depression Constant: This property allows for measurable changes in the freezing point with small amounts of solute, enhancing precision.
• Cost-Effectiveness: As an inexpensive substance, it allows scientists to conduct experiments without significant financial constraints.
• High Melting Point: This feature prevents it from evaporating at room temperature, ensuring stable experimental conditions.

These combined properties make camphor an ideal choice for molecular weight determination using the Rast method, contributing to both the accuracy and efficiency of measurements.