Question

Freezing-point depression is measured by (a) Beckmann's method (b) Rast's camphor method (c) both (d) none of these

Short answer

The correct answer is (c) both

Step-by-step solution

Understanding the Methods

Beckmann's method is primarily used to determine the molecular weights of organic substances by freezing point depression. Similarly, in Rast's camphor method, the molecular weight of smaller quantities of any solute is determined using camphor as the solvent. Both are used for measuring freezing point depression.

Matching with the Answers

With the knowledge from step 1, it's clear that both methods are used to measure freezing-point depression. Matching this information with given options, the correct answer is the one stating that both methods are used.

Key concepts

Understanding Freezing-Point Depression

Freezing-point depression is a colligative property of solutions, which means that it is affected by the number of solute particles in a solvent, regardless of their identity. When a solute is dissolved in a solvent, the freezing point of the solution is lower than that of the pure solvent. This phenomenon is due to the presence of solute particles, which interfere with the orderly crystallization process necessary for a substance to solidify, or freeze. As a result, more energy (a lower temperature) is required to overcome this interference and achieve solidification. Freezing-point depression is exploited in methods such as the Beckmann's and Rast's camphor method to determine the molecular weight of substances.

When studying freezing-point depression, it is essential to understand that it is a particular and reliable property that can be quantified. The amount by which the freezing point is lowered is proportional to the molality (moles of solute per kilogram of solvent) of the solution. This relationship allows scientists to calculate the molecular weight of the solute by measuring the depression of the freezing point.

Beckmann's Method Explained

Beckmann's method is a classical approach designed to find the molecular weight of organic substances. It involves dissolving a known weight of substance in a solvent and carefully measuring the depression of the freezing point of the solution. The Beckmann thermometer, known for its precision, is typically used in this method to measure temperature changes to within a hundredth of a degree Celsius.

One vital aspect of Beckmann's method is the accurate calibration of the freezing point of the pure solvent before dissolving the solute. After adding the solute, the freezing point is measured again to determine the difference. By applying the formula involving the depression of the freezing point, the known mass of the solute, and the solvent's cryoscopic constant (a property that represents the freezing-point depression of the solvent by a one-molar solution of a non-volatile solute), chemists can calculate the molecular weight of the solute.

Rast's Camphor Method Demystified

Rast's camphor method is another established technique for determining the molecular weight of a solute, albeit for smaller quantities. It employs camphor as the solvent due to its beneficial characteristics, such as a high molecular weight, low freezing point, and ability to form crystalline solids with many organic compounds. This method proves particularly useful when the solute is available only in minimal amounts.

Similar to Beckmann's method, the Rast's camphor method follows the principle of measuring the freezing-point depression caused by the introduction of a foreign substance into the camphor. The procedure focuses on finely powdered camphor due to its quick dissolution and rapid establishment of equilibrium. Once the solute dissolves in the camphor, the depression in the freezing point is recorded, providing data that correlates to the molecular weight of the solute. The calculations in Rast's camphor method involve using the molal depression constant specific to camphor.

The Role of Solvent Properties

The success of both the Beckmann's and Rast's camphor method in determining molecular weights relies heavily on the properties of the solvents used. A solvent should ideally have a known, consistent freezing point to ensure accurate measurements. Other important solvent properties include non-reactivity with the solute, a relatively high cryoscopic constant for greater sensitivity in detecting freezing-point changes, and the ability to dissolve the solute completely without undergoing any chemical changes itself.

Solvents must also be pure and free from impurities that can alter the freezing point, shape the formation of the crystalline structure, or impact the solute's solubility. For example, the selection of camphor in Rast's method is due to its specific properties that favor molecular weight determinations, such as lower volatility and a precise, well-characterized freezing point, providing a stable basis for the sensitive measurements required in freezing-point depression methods.