Camphor is used as a solvent to determine molecular weight of non-volatile solute by Rast method because for camphor:

Its molal depression constant is high

The correct answer is option 1. Its molal depression constant is high.

Let us delve into why camphor is used in the Rast method for determining the molecular weight of a non-volatile solute and why the high molal depression constant is particularly important.

1. Its molal depression constant is high:

The molal depression constant, \( K_f \), is a measure of how much the freezing point of a solvent decreases when a non-volatile solute is dissolved in it. It is defined for a particular solvent and reflects the solvent's ability to lower its freezing point for each mole of solute added. A high \( K_f \) value means that the solvent will exhibit a significant decrease in freezing point for a given concentration of solute. This large change makes it easier to measure the depression accurately, which in turn allows for more precise determination of the molecular weight of the solute. For accurate molecular weight determination, it is crucial to have a solvent that shows a pronounced change in freezing point with the addition of solute, which is achieved by using a solvent with a high \( K_f \).

2. High Melting Point:

Camphor has a high melting point (about 179°C). This high melting point is advantageous because it ensures that camphor remains in a solid state over a wide range of temperatures, including at room temperature and even when slightly heated if needed. This stability is essential for accurate measurement and prevents the solvent from melting or changing its phase during the experiment.

3. Not Cheap:

While camphor may not be the cheapest solvent available, its selection for the Rast method is more related to its physical and chemical properties rather than its cost. The effectiveness of a solvent in the Rast method is primarily determined by its \( K_f \) and melting point rather than its price.

Summary:

In summary, camphor is chosen for the Rast method due to its high molal depression constant, which ensures a significant and measurable change in freezing point when a solute is dissolved. This property enhances the accuracy of molecular weight determination. The high melting point of camphor also contributes to its suitability by ensuring it remains solid during the experiment. While cost is a consideration, it is not the primary reason for selecting camphor.

The primary reason camphor is used in the Rast method is because: Its molal depression constant is high.