Problem 11 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

The molecular mass of a volatile substance is determined by Victor Meyer's method.

Step 1: Understanding the Question

The exercise asks about determining the molecular mass of a volatile substance. We need to identify the correct method from the given options that is typically used for this purpose.

Step 2: Evaluating the Methods

Let's briefly review each method: (a) Liebig's Method: Primarily used for determining the percentage composition of carbon and hydrogen in organic compounds. (b) Kjeldahl's Method: Used for determining nitrogen content in organic substances. (c) Duma's Method: Mostly used for the determination of nitrogen in compounds. (d) Victor Meyer's Method: Specifically designed to determine the molecular weight of volatile liquids by measuring the volume of vapor they displace.

Step 3: Identifying the Correct Method

Among the options, Victor Meyer's method is specifically designed to find the molecular mass of volatile substances by utilizing the displacement of vapor. This matches the requirement of the exercise.

Key Concepts

Understanding Molecular Mass

Molecular mass is a fundamental concept in chemistry that represents the mass of a single molecule of a substance. It is determined by summing up the atomic masses of all the atoms present in the molecule's formula. The molecular mass is usually expressed in unified atomic mass units (u) or Daltons (Da). For example, the molecular mass of water (H₂O) is calculated by adding the atomic masses of two hydrogen atoms and one oxygen atom.

Determining the molecular mass of a substance is crucial for:

• Understanding its properties and behavior.
• Conducting stoichiometric calculations in chemical reactions.
• Identifying unknown substances through methods like Victor Meyer's method.

This concept is particularly important when dealing with volatile substances, as their molecular mass helps in characterizing and differentiating them.

Characteristics of Volatile Substances

Volatile substances are compounds that can vaporize easily at normal temperatures due to their low boiling points. They are typically characterized by:

• High vapor pressure at room temperature.
• Low molecular interactions, which allow them to transition to the gaseous phase more readily.
• Suitability for methods like Victor Meyer's method, which takes advantage of their ability to easily convert to vapor.

Volatile substances are prevalent in both nature and industry. Examples include gasoline, alcohol, and essential oils. Understanding their volatility is crucial for applications such as:

• Perfume manufacturing, where controlled evaporation is needed.
• Gas chromatography, for separating and analyzing mixtures.
• Safety measures, as volatile substances can be flammable and hazardous.

Comprehending the behavior and properties of volatile substances allows chemists to accurately determine their molecular mass using specialized methods.

Chemistry Methods: Victor Meyer's Method

Victor Meyer's method is a classical technique developed to accurately determine the molecular mass of volatile substances. This method involves measuring the volume of vapor displaced by a known mass of the substance. It is widely used because:

• It eliminates the need for precise temperature and pressure control.
• It can be adapted for various types of volatile liquids.
• It provides reliable results if performed correctly.

The procedure involves heating a known mass of the volatile substance in a Victor Meyer tube, which is placed in a bath of a liquid with a much higher boiling point, often water or oil. As the substance vaporizes, it displaces an equal volume of air or another gas, which is then measured. Using the ideal gas law, the molecular mass of the substance can be calculated from this volume displacement.

This method demonstrates the ingenious approach chemists use to exploit the physical properties of substances to obtain precise measurements, showcasing the innovative nature of chemical experimentation.