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

Short answer

The correct answer is (d) all of these.

Step-by-step solution

Understanding the Problem

We are tasked with identifying why camphor is used as a solvent in the Rast method for determining molecular weight. The problem offers four options to choose from, where multiple reasons might be correct.

Analyzing Option (a)

Option (a) suggests that camphor's molal depression constant is high. A high molal depression constant is desirable in the Rast method as it allows for a more significant change in melting point with the addition of solute, making the molecular weight determination more precise.

Analyzing Option (b)

Option (b) claims camphor is very cheap. Camphor is indeed inexpensive, which makes it a cost-effective choice for experiments where it is used in larger quantities.

Analyzing Option (c)

Option (c) states camphor has a high melting point. A high melting point ensures that the solvent remains solid over a wide range of temperatures, which is beneficial during the Rast method as it minimizes errors due to temperature variations.

Evaluating Option (d)

Option (d) suggests that all the given reasons (a, b, and c) are correct. Given that we have verified all options (a), (b), and (c) are individually correct, this implies that option (d) is the most comprehensive choice.

Final Decision

After analyzing all statements and confirming their correctness, the logical choice is option (d), as it encompasses all the valid reasons for using camphor in the Rast method.

Key concepts

Molecular Weight Determination

Determining the molecular weight of a solute is a vital component in chemistry, which helps in understanding the properties and behavior of a substance. One reliable method used for this purpose is the Rast method. This technique involves measuring the depression in the melting point of a solvent when a non-volatile solute is added to it. The change in temperature is used to calculate the molecular weight of the solute. In the Rast method, it's crucial to have a significant depression in the melting point to obtain precise results. This is why camphor, with its high molal depression constant, is often used. This enables chemists to detect measurable changes that provide an accurate calculation of molecular weight. By mixing a known mass of the non-volatile solute into the camphor, and observing the decrease in melting point, they can determine the molecular weight using the formula:\[\Delta T_f = K_f \cdot m\]where \(\Delta T_f\) is the freezing point depression, \(K_f\) is the molal depression constant of camphor, and \(m\) is the molality of the solution.

Camphor

Camphor is a solid, waxy, and aromatic compound extracted from the wood of the camphor laurel trees. It's known for its strong scent and has various applications beyond being a solvent in the Rast method. There are several reasons why camphor is an excellent choice for molecular weight determination:

• High Molal Depression Constant: This characteristic ensures a significant change in the melting point with the addition of solute, enhancing precision in calculations.
• High Melting Point: Camphor remains stable over a wide range of temperatures, which minimizes errors during melting point measurement.
• Cost-Effective: Being inexpensive makes camphor a practical choice for numerous experiments.

These properties make camphor indispensable in labs where determining the molecular weight of non-volatile solutes is essential.

Molal Depression Constant

The molal depression constant, symbolized as \(K_f\), is a fundamental factor in the study of colligative properties. It quantifies the depression of the freezing point of a solvent upon the addition of a solute. A high molal depression constant is desirable in experiments, like those involving the Rast method, because it will produce a larger change in the freezing point. This sensitivity improves the accuracy of molecular weight calculations. The mathematical relationship is given by the equation:\[\Delta T_f = K_f \cdot m\]where \(\Delta T_f\) is the change in freezing point, \(K_f\) is the molal depression constant, and \(m\) is the molality. In the context of camphor, its high \(K_f\) ensures clear, measurable changes in melting and freezing points, leading to precise experimental outcomes.

Non-Volatile Solute

In the context of chemistry and molecular weight determination, a non-volatile solute is a substance that does not easily vaporize. This property is crucial when determining molecular weight using techniques like the Rast method. By nature, non-volatile solutes do not evaporate or boil off when the solvent is heated. This stability ensures the measurement focuses solely on the change in melting point rather than the loss of solute through evaporation. For instance, when a non-volatile solute is dissolved in camphor, it leads to a depression in the melting point without the complication of the solute being lost to vaporization. This allows for more accurate molecular weight calculations, critical for reliable scientific results. Using non-volatile solutes reduces errors and increases the precision of experiments surrounding the determination of their molecular masses.