Question

Which of the following is an extensive property? (a) Molarity of substance (b) Number of moles (c) Molar volume of substance (d) Mole fraction of substance

Short answer

(b) Number of moles is the extensive property.

Step-by-step solution

Understand the Definition of an Extensive Property

Extensive properties depend on the amount of substance present. Common examples include mass, volume, and number of moles. These properties change when the quantity of matter changes.

Analyze Option (a) - Molarity of Substance

Molarity is defined as the number of moles of solute per liter of solution. Since it is dependent on volume and doesn't change with the amount of solute alone but rather its concentration, molarity is an intensive property.

Analyze Option (b) - Number of Moles

The number of moles is a direct measure of the quantity of matter and changes when the amount of substance changes. Therefore, it is an extensive property.

Analyze Option (c) - Molar Volume of Substance

Molar volume is the volume occupied by one mole of a substance. It is independent of the quantity of substance present and remains constant for a given substance under specified conditions, making it an intensive property.

Analyze Option (d) - Mole Fraction of Substance

Mole fraction is a ratio that expresses the proportion of moles of one component to the total moles in a mixture. As it is a ratio, it remains constant regardless of the sample size, making it an intensive property.

Identify the Correct Extensive Property

Based on the analysis, the number of moles is the property that varies with the amount of substance, identifying it as the extensive property among the given options.

Key concepts

Intensive Property

Intensive properties are characteristics of substances that do not depend on the amount of substance present. Unlike extensive properties, which change if you alter the quantity of the substance, intensive properties remain the same, regardless of scale. This makes them particularly useful when studying substances since they are consistent and reliable for identification purposes.
Examples of intensive properties include:

• Temperature - The temperature of a substance remains constant regardless of its total quantity.
• Density - No matter how much you have, the density of a gram of gold is the same as a ton of it.
• Boiling and melting points - These points stay consistent for a pure substance in certain conditions, regardless of sample size.
• Molarity - As explained earlier, it's about concentration and not quantity.

Understanding these properties can significantly help in the study and application of physical chemistry. They allow scientists to identify substances and predict how they will behave under various conditions without having to measure or alter the amount of the material.

Number of Moles

The concept of "number of moles" is pivotal in chemistry, as it provides a bridge between the atomic scale and the macroscopic scale. Defined simply, a mole is a unit that relates to Avogadro's number, which is approximately \(6.022 \times 10^{23}\). This number represents the quantity of atoms or molecules in one mole of a substance.
In practice, the number of moles gives us a clear measure of how much of a substance we have. Since it is an extensive property, the number of moles changes depending on the amount of substance. Here are some key points:

• Moles allow for the quantification of substance in chemical reactions.
• It helps in converting between grams and atomic or molecular amounts using molar mass.
• It's essential for determining concentrations like molarity, which is the number of moles per liter.
• It facilitates understanding of reaction stoichiometry, which involves balancing chemical equations.

The quantity of moles directly impacts reactions and properties that rely on the amount of substance, making it fundamental to the study and application of chemistry.

Physical Chemistry Concepts

Physical chemistry focuses on understanding the physical properties and transformations of substances. This branch of chemistry applies physics principles to chemical systems, providing deep insights into how molecular and atomic compositions translate to physical properties.
Key concepts central to physical chemistry include:

• Thermodynamics - This explores energy transformations and how they affect matter, foundational for processes like reaction rates and equilibrium.
• Kinetics - This study investigates how quickly reactions occur and the factors influencing these speeds, crucial for both theoretical and practical applications.
• Quantum Chemistry - This is the application of quantum mechanics in understanding molecular structures and behavior, providing a microscopic view of chemical processes.
• Equilibrium - This refers to the state where reactants and products remain constant over time, essential for processes involving reversible reactions.

By integrating these principles, physical chemistry unravels complex chemical processes and helps predict the behavior of materials under a range of conditions. This scientific field plays a critical role in developing new materials, understanding biological processes, and advancing technological innovations.