Question

Molecular mass of a volatile substance may be obtained by: (a) Liebig's method (b) Kjeldhal's method (c) Duma's method (d) Victor Meyer's method

Short answer

The method for determining the molecular mass of a volatile substance is Victor Meyer's method (option d).

Step-by-step solution

Understanding the Methods

First, we need to understand each method mentioned to determine which one pertains to measuring molecular mass. Liebig's method is used for determining the carbon content in organic compounds. Kjeldahl's method measures nitrogen content in samples. Dumas' method is used for nitrogen analysis. Victor Meyer's method is used to determine the vapor density of volatile substances which is crucial in determining molecular mass.

Connecting Victor Meyer's Method

Victor Meyer's method specifically helps in estimating the molecular mass of a volatile substance through the measurement of vapor density. This method involves vaporizing the substance and calculating its vapor density, which can then be used to determine the molecular mass.

Identifying the Correct Option

Given the methods and their purposes, Victor Meyer's method (option d) is tailored for determining the molecular mass of volatile substances by measuring vapor density. It is the method directly linked to measuring molecular mass when dealing with volatile substances.

Key concepts

Determination of Molecular Mass

The molecular mass, also known as molecular weight, is a fundamental property of a substance. It is defined as the mass of a given molecule. Calculating this is important in chemistry because it helps in understanding how much of one substance is needed to react with a given amount of another. It plays a crucial role in stoichiometric calculations.

To determine the molecular mass of a volatile substance, we employ methods that take advantage of the substance's gaseous state. One common approach is using the ideal gas law, which relates the physical properties of gases: pressure, volume, temperature, and the number of moles. The formula used is, \\[ PV = nRT \]

Where:

• \( P \) is the pressure,
• \( V \) is the volume,
• \( n \) is the number of moles,
• \( R \) is the ideal gas constant,
• \( T \) is the absolute temperature.

By measuring these variables, the molecular mass can be derived when the vapor density is known, since it is proportional to the molecular mass of the gas.

Vapor Density Measurement

Vapor density is a term used to describe the ratio of the mass of a given volume of gas compared to the mass of the same volume of hydrogen. It is typically expressed as \\[ \text{Vapor Density} = \frac{\text{Molecular Mass}}{2} \] \
This relationship provides a bridge to determine the molecular mass once the vapor density is known. Vapor density measurement often involves the use of Victor Meyer's apparatus, which is specifically designed to find the density of volatile liquids.

The method is relatively simple. A known quantity of a volatile substance is vaporized, and the vapor displaces an equal volume of air that is then measured. This helps in determining the vapor density, and subsequently, the molecular mass. The apparatus used ensures that the vaporization occurs at a constant temperature, which is crucial for accurate results.

Volatile Substances Analysis

Volatile substances are those that easily vaporize at relatively low temperatures. Analyzing these substances requires special methods because their gaseous state can lead to quick dispersion. Hence, traditional weighing techniques cannot be used. Instead, scientists rely on the measurement of properties of gases.

Victor Meyer's Method is particularly effective for such analyses. It involves heating the volatile substance within a specially designed flask. The flask displaces an amount of air equal to the vapor produced by the substance. This displacement is proportional to the density of the vapor, which is then used to calculate the molecular mass. Since the method avoids direct handling of the vapor, it reduces potential errors that could affect the accuracy of results.

• Ensures safe handling of gases.
• Provides accurate measurement of vapor density, which is crucial for determining molecular mass.
• Widely applicable for laboratory settings dealing with various volatile substances.