Lesson Plan | Traditional Methodology | Colligative Properties: Osmotic Pressure

Objectives

Duration: (10 - 15 minutes)

The purpose of this stage is to ensure that students clearly understand what will be covered in the class and what the main objectives are. This will help guide the students' focus during the explanation and problem solving, ensuring that they internalize the fundamental concepts and know how to apply them in practical situations.

Main Objectives

1. Understand the concept of osmotic pressure and its relevance in colligative properties.

2. Learn the osmotic pressure equation and its variables.

3. Enable students to solve problems involving calculations of osmotic pressure in solutions.

Introduction

Duration: (10 - 15 minutes)

The purpose of this stage is to ensure that students clearly understand what will be covered in the class and what the main objectives are. This will help guide the students' focus during the explanation and problem solving, ensuring that they internalize the fundamental concepts and know how to apply them in practical situations.

Context

Start the class by introducing the concept of osmotic pressure. Explain that osmotic pressure is the pressure required to stop the flow of solvent through a semipermeable membrane separating two solutions with different solute concentrations. Use a daily example, like the process of water purification by reverse osmosis, to make the concept more tangible. Detail how osmotic pressure is one of the colligative properties, which depend on the number of solute particles in the solution and not on their nature.

Curiosities

Did you know that osmotic pressure is fundamental for cell survival? Cells use osmotic pressure to maintain the balance of water and nutrients inside them. Without osmotic pressure, cells could swell until they burst or shrivel depending on external conditions. This phenomenon is the same that occurs when a plant cell is placed in a saline solution: it loses water and shrivels.

Development

Covered Topics

1. Concept of Osmotic Pressure: Explain that osmotic pressure is the pressure required to prevent osmosis, that is, the movement of solvent through a semipermeable membrane to equalize solute concentrations on both sides. 2. Osmotic Pressure Equation: Introduce Van't Hoff's formula for osmotic pressure: π = MRT, where π is the osmotic pressure, M is the molarity of the solution, R is the universal gas constant (0.0821 L·atm·K⁻¹·mol⁻¹), and T is the temperature in Kelvin. Explain each variable and how they relate.

Questions Discussion

Duration: (20 - 25 minutes)

The purpose of this stage is to review and consolidate the knowledge acquired by the students, ensuring that they understand the concepts well and know how to apply them in problem solving. Through detailed discussion of the answers, students will be able to identify possible mistakes and clarify doubts, promoting a deeper and more meaningful learning experience.

Discussion

• 1. Calculate the osmotic pressure of an aqueous glucose (C₆H₁₂O₆) solution at 25°C, knowing that the molarity of the solution is 0.1 mol/L. (R = 0.0821 L·atm·K⁻¹·mol⁻¹)

To calculate the osmotic pressure, we use Van't Hoff's formula: π = MRT.

• M (molarity) = 0.1 mol/L
• R (gas constant) = 0.0821 L·atm·K⁻¹·mol⁻¹
• T (temperature in Kelvin) = 25°C + 273 = 298K

Substituting the values: π = 0.1 * 0.0821 * 298 = 2.45 atm.

Therefore, the osmotic pressure of the solution is 2.45 atm.

• 2. A solution contains 0.5 mol of solute in 2 liters of solvent. Calculate the osmotic pressure of this solution at 30°C. (R = 0.0821 L·atm·K⁻¹·mol⁻¹)

First, we calculate the molarity (M) of the solution:

• M = amount of solute (mol) / volume of solution (L) = 0.5 mol / 2 L = 0.25 mol/L

Now, we use Van't Hoff's formula: π = MRT.

• M = 0.25 mol/L
• R = 0.0821 L·atm·K⁻¹·mol⁻¹
• T = 30°C + 273 = 303K

Substituting the values: π = 0.25 * 0.0821 * 303 = 6.22 atm.

Therefore, the osmotic pressure of the solution is 6.22 atm.

• 3. Determine the molarity of a solution whose osmotic pressure is 2.5 atm at 20°C. (R = 0.0821 L·atm·K⁻¹·mol⁻¹)

To find the molarity (M), we rearrange Van't Hoff's formula: M = π / (RT).

• π = 2.5 atm
• R = 0.0821 L·atm·K⁻¹·mol⁻¹
• T = 20°C + 273 = 293K

Substituting the values: M = 2.5 / (0.0821 * 293) = 0.104 mol/L.

Therefore, the molarity of the solution is 0.104 mol/L.

Student Engagement

1.  Questions for Reflection and Discussion: 2. 1. How does temperature affect osmotic pressure? Give practical examples. 3. 2. Why is osmotic pressure considered a colligative property? 4. 3. How is osmotic pressure important for biological processes in cells? 5. 4. How is reverse osmosis used in water desalination? Explain the process. 6. 5. What are the possible biological consequences if osmotic pressure is not properly maintained in living organisms?

Conclusion

Duration: (10 - 15 minutes)

The purpose of this stage is to review and consolidate the main concepts covered in the class, ensuring that students have a clear and complete understanding of the content. It also reinforces the connection between theory and its practical applications, highlighting the relevance of the topic for everyday life and various fields of knowledge.

Summary

• Concept of osmotic pressure and its importance in colligative properties.
• Van't Hoff's equation for calculating osmotic pressure: π = MRT.
• The influence of solute concentration and temperature on osmotic pressure.
• Practical applications of osmotic pressure in processes like reverse osmosis and cellular biology.

The lesson connected theory with practice by presenting everyday examples, such as water purification by reverse osmosis, and discussing the importance of osmotic pressure in biological processes, such as maintaining the internal pressure of cells. This helped students understand how theoretical concepts apply in real and practical situations.

The topic presented is of great practical importance, as osmotic pressure is crucial for cell survival and for industrial processes such as water desalination. Understanding osmotic pressure helps comprehend everyday phenomena, such as the effect of saline solutions on cells and the technology used to obtain drinking water from salt water.