What is Relative Lowering of Vapour Pressure?

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Grade Level:

Class 12

AI/ML, Physics, Biotechnology, FinTech, EVs, Space Technology, Climate Science, Blockchain, Medicine, Engineering, Law, Economics

Definition

What is it?

Relative Lowering of Vapour Pressure is a colligative property that describes how adding a non-volatile solute to a solvent reduces the solvent's vapour pressure, and this reduction is relative to the pure solvent's vapour pressure. Simply put, it's the fraction by which the vapour pressure goes down when you mix something into a liquid.

Simple Example

Quick Example

Imagine you have a glass of water (pure solvent). Some water molecules escape as vapour. Now, if you add sugar (non-volatile solute) to the water, fewer water molecules can escape because some sugar molecules are blocking the surface. This means the 'pressure' of the water vapour above the glass will be lower. The 'relative lowering' is how much this pressure drops compared to the original pure water.

Worked Example

Step-by-Step

Let's calculate the relative lowering of vapour pressure for a solution.

STEP 1: Identify the vapour pressure of the pure solvent (P°). Let's say pure water has a vapour pressure of 20 mmHg at a certain temperature.

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STEP 2: Identify the vapour pressure of the solution (P_s). After dissolving some glucose, the solution's vapour pressure becomes 18 mmHg.

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STEP 3: Calculate the lowering of vapour pressure (ΔP). This is P° - P_s. So, ΔP = 20 mmHg - 18 mmHg = 2 mmHg.

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STEP 4: Calculate the relative lowering of vapour pressure. This is ΔP / P°. So, Relative Lowering = 2 mmHg / 20 mmHg.

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STEP 5: Simplify the fraction. Relative Lowering = 1/10 or 0.1.

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The relative lowering of vapour pressure for this solution is 0.1.

Why It Matters

Understanding relative lowering of vapour pressure is crucial in fields like Biotechnology for designing purification processes and in Medicine for understanding how different solutions behave in the body. It helps engineers in chemical industries create products with specific properties and even helps climate scientists understand humidity.

Common Mistakes

MISTAKE: Confusing 'lowering of vapour pressure' with 'relative lowering of vapour pressure'. | CORRECTION: Lowering of vapour pressure (ΔP) is P° - P_s. Relative lowering is (P° - P_s) / P°, which is a fraction or ratio.

MISTAKE: Forgetting that the solute must be non-volatile for this concept to apply directly. | CORRECTION: Relative lowering of vapour pressure primarily applies when a non-volatile solute is added, meaning the solute itself doesn't easily turn into vapour.

MISTAKE: Using the vapour pressure of the solute in calculations instead of the solvent. | CORRECTION: The calculations always refer to the vapour pressure of the solvent, as the non-volatile solute does not contribute significantly to the vapour pressure.

Practice Questions

Try It Yourself

QUESTION: The vapour pressure of pure benzene is 75 mmHg. When a non-volatile solute is added, the vapour pressure of the solution becomes 70 mmHg. Calculate the lowering of vapour pressure. | ANSWER: Lowering of vapour pressure = 75 - 70 = 5 mmHg.

QUESTION: Using the data from Q1 (pure benzene P° = 75 mmHg, solution P_s = 70 mmHg), calculate the relative lowering of vapour pressure. | ANSWER: Relative lowering = (75 - 70) / 75 = 5 / 75 = 1/15 or approximately 0.067.

QUESTION: A solution containing a non-volatile solute has a vapour pressure of 60 mmHg. If the relative lowering of vapour pressure is 0.2, what was the vapour pressure of the pure solvent? | ANSWER: Let P° be the pure solvent's vapour pressure. (P° - 60) / P° = 0.2. So, P° - 60 = 0.2 * P°. This means 0.8 * P° = 60. Therefore, P° = 60 / 0.8 = 75 mmHg.

MCQ

Quick Quiz

Which factor does Relative Lowering of Vapour Pressure depend on?

Nature of the solvent only

Nature of the solute only

Concentration of the solute only

Both nature of solvent and solute concentration

The Correct Answer Is:

Relative lowering of vapour pressure is a colligative property, meaning it depends only on the number of solute particles (concentration) and not on their chemical identity (nature of solute).

Real World Connection

In the Real World

This concept is used when developing anti-freeze solutions for car radiators, especially in colder regions of India like Himachal Pradesh or Jammu & Kashmir. Adding a non-volatile solute like ethylene glycol lowers the freezing point and also the vapour pressure of water, making the solution more stable across temperature ranges. It also helps in food preservation by controlling moisture.

Key Vocabulary

Key Terms

VAPOUR PRESSURE: The pressure exerted by the vapour in equilibrium with its liquid phase. | NON-VOLATILE SOLUTE: A substance that does not easily turn into a gas at normal temperatures. | COLLIGATIVE PROPERTY: A property of solutions that depends only on the number of solute particles, not on their identity. | SOLVENT: The substance in which a solute is dissolved to form a solution.

What's Next

What to Learn Next

Next, you should learn about Raoult's Law, which provides a mathematical relationship for calculating vapour pressure lowering. It builds directly on this concept and helps you predict how much the vapour pressure will drop based on the solute's amount. Keep exploring!