Lesson Plan | Traditional Methodology | Concentration Units: Molality

Objectives

Duration: (10 - 15 minutes)

This stage aims to present the main objectives of the lesson to the students, emphasizing what will be learned and the skills that will be developed throughout the lesson. By clearly defining the objectives, students will have a clear understanding of what is expected of them and what they should focus on during the explanation and practical exercises.

Main Objectives

1. Understand the concept of molality and its importance in chemistry.

2. Learn the formula for molality and how to apply it in calculations.

3. Develop skills to solve problems related to the molality of solutions.

Introduction

Duration: (10 - 15 minutes)

This stage is essential to capture the attention of the students and prepare them for the content to be addressed. By presenting the context and curiosities, the teacher sparks students' interest and demonstrates the practical relevance of the topic. This facilitates the understanding and retention of the information that will be taught throughout the lesson.

Context

To start the lesson on molality, it is important to situate students in the general context of concentration units. Explain that in chemistry, understanding how to measure the concentration of solutions is fundamental for various applications, from industrial processes to drug formulation. Molality is one of these units and is especially useful in situations where temperature can vary, as it does not depend on the volume of the solution, but rather on the mass of the solvent.

Curiosities

Did you know that molality is a concentration unit that remains constant even with temperature changes? This makes it ideal for chemical studies that involve thermal variations, such as endothermic and exothermic reactions. For example, in the production processes of pharmaceutical products, precision in concentration is crucial to ensure the efficacy and safety of medications.

Development

Duration: (40 - 50 minutes)

This stage aims to deepen students' understanding of molality, ensuring they not only comprehend the theory but also know how to apply it in practical calculations. By addressing specific topics and solving questions in class, students will be able to consolidate knowledge and develop skills to solve problems related to the molality of solutions.

Covered Topics

1. Definition of Molality: Explain that molality (m) is a measure of concentration defined as the number of moles of solute per kilogram of solvent. The formula that represents molality is: m = n / m_solvent, where n is the number of moles of the solute and m_solvent is the mass of the solvent in kilograms. 2. Importance of Molality: Detail that molality is especially useful in situations where temperature varies, as opposed to other concentration units, it does not depend on the volume of the solution. This makes it ideal for studies of chemical reactions involving thermal variations. 3. Practical Examples of Molality Calculation: Present practical examples to illustrate the calculation of molality. For instance, calculate the molality of a solution where 5 moles of NaCl are dissolved in 2 kg of water. Demonstrate step by step how to apply the molality formula. 4. Comparison with Other Concentration Units: Compare molality with other concentration units, such as molarity and mole fraction, highlighting the differences and advantages of molality in certain contexts. 5. Applications of Molality in Real Problems: Explain how molality is used in practical situations, such as in the manufacturing of pharmaceutical products where precision in concentration is crucial, and in studies of endothermic and exothermic reactions.

Classroom Questions

1. Calculate the molality of a solution containing 3 moles of KCl dissolved in 1.5 kg of water. 2. A solution is prepared by dissolving 10 moles of glucose (C6H12O6) in 5 kg of water. What is the molality of this solution? 3. If 2 moles of H2SO4 are dissolved in 0.8 kg of water, what is the molality of the resulting solution?

Questions Discussion

Duration: (20 - 25 minutes)

The purpose of this stage is to ensure that students fully understand the calculations of molality and its practical applications. By discussing the resolved questions in detail and engaging students with reflective questions, the teacher consolidates learning and clarifies potential doubts, promoting a deeper and more lasting understanding of the content.

Discussion

• Question 1: Calculate the molality of a solution that contains 3 moles of KCl dissolved in 1.5 kg of water.

• Explanation: To calculate the molality (m), the formula m = n / m_solvent is used, where n is the number of moles of the solute and m_solvent is the mass of the solvent in kilograms. Substituting the given values, we have m = 3 mol / 1.5 kg = 2 mol/kg. Therefore, the molality of the solution is 2 mol/kg.

• Question 2: A solution is prepared by dissolving 10 moles of glucose (C6H12O6) in 5 kg of water. What is the molality of this solution?

• Explanation: Using the formula m = n / m_solvent, we substitute the given values: m = 10 mol / 5 kg = 2 mol/kg. Therefore, the molality of the solution is 2 mol/kg.

• Question 3: If 2 moles of H2SO4 are dissolved in 0.8 kg of water, what is the molality of the resulting solution?

• Explanation: Again, we apply the formula m = n / m_solvent. Substituting the given values, we have m = 2 mol / 0.8 kg = 2.5 mol/kg. Therefore, the molality of the solution is 2.5 mol/kg.

Student Engagement

1. 📝 Questions and Reflections: 2. Why is molality a useful concentration unit in situations where temperature varies? 3. How does molality compare with other concentration units, such as molarity? 4. In what practical scenarios is molality preferred over other concentration units? 5. If the amount of solvent in a solution increases, how does that affect the molality of the solution? 6. What would be the molality if 0.5 moles of solute were dissolved in 0.25 kg of solvent?

Conclusion

Duration: (10 - 15 minutes)

The purpose of this stage is to recap the main points addressed during the lesson, reinforcing the students' understanding. Furthermore, by connecting theory with practice and demonstrating the relevance of the topic to everyday life, this stage helps consolidate learning and highlight the importance of the presented content.

Summary

• Introduction to the concept of molality and its definition as the ratio of the number of moles of solute to the mass in kilograms of solvent.
• Discussion of the importance of molality in situations where temperature varies, highlighting its independence from the volume of the solution.
• Practical examples of molality calculations, including step-by-step resolution of specific problems.
• Comparison of molality with other concentration units, such as molarity and mole fraction.
• Practical applications of molality in contexts such as pharmaceutical manufacturing and studies of endothermic and exothermic reactions.

During the lesson, the theory of molality was connected with practice through concrete examples and problem-solving. Students were able to see how to apply the molality formula in real calculations, understanding its utility in practical situations and its advantage over other concentration units when temperature varies.

The study of molality is crucial for various fields of knowledge and industry. For example, in pharmacology, precision in the concentration of solutions is vital for the efficacy and safety of medications. Additionally, molality is particularly useful in studies of chemical reactions involving thermal variations, which is common in many industrial and laboratory processes.