Class: Senior Secondary School 1 (SS1, SS 1, SSS1, SSS 1)
Term: 2nd Term
Week: 8
Age: 15 years
Duration: 45 minutes
Subject: Chemistry
Curriculum Theme: Chemistry
Previous Lesson: Gas Laws: Boyle’s Law, Charles’ Law and General Gas Equation.
Topic: Gas Laws
Subject Matter: Graham’s law of diffusion, molar volume of gases, Avogadro’s number and the mole concept, calculations based on gas laws and PV = nRT

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

Define diffusion and state Graham’s law of diffusion.
State Avogadro’s law and Gay-Lussac’s law.
Explain the meaning of mole and Avogadro’s number.
State the molar volume of a gas at standard temperature and pressure (STP).
Use PV = nRT to calculate unknown gas variables in simple problems.

Affective Domain:

Show interest in observing and explaining diffusion in gases.
Appreciate careful handling of laboratory chemicals and safety rules.

Psychomotor Domain:

Observe the diffusion demonstration and describe the result correctly.
Arrange simple data and substitute correctly into PV = nRT during calculations.

Work in pairs or groups to discuss observations and solve calculation questions.
Communicate solutions and explanations clearly to the class.

Reference Materials

The following resources were used in planning this lesson:

9 Years Basic Education Curriculum
State Unified Scheme of Work
New School Chemistry for Senior Secondary Schools (or any approved SS Chemistry textbook)
Encyclopaedia Britannica: Diffusion
Encyclopaedia Britannica: Avogadro’s Number
Khan Academy: Ideal Gas Law (PV = nRT)

Instructional Materials

The teacher will teach this lesson with the aid of:

Cotton wool soaked in ammonia solution
Cotton wool soaked in concentrated hydrochloric acid (HCl)
Glass tube or glass vessel with covers
Thermometer
Charts showing gas law relationships and sample calculations
Calculator, board/marker, and prepared question set

Rationale for the Lesson

This lesson helps pupils understand how gases spread, react, and occupy space, which relates to daily experiences like smell spreading in a room. It also improves their ability to use the mole concept and gas equations for calculations in chemistry.

Prerequisite/Previous Knowledge

Pupils have learned basic gas properties, the meanings of pressure, volume, and temperature, and can perform simple calculations and unit conversions.

Lesson Content/Board Summary

Gas Laws (Diffusion, Mole Concept, and Gas Calculations)

Meaning of Diffusion

Diffusion is the movement of gas particles from a region of higher concentration to a region of lower concentration until they are evenly spread.

The following are characteristics of diffusion in gases:

It occurs because gas particles are in constant random motion.
It is faster in gases than in liquids and solids.
It increases with higher temperature.
It depends on the mass of gas particles.

Graham’s Law of Diffusion

Graham’s law states that at the same temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass.

The following are the mathematical statements of Graham’s law:

Rate of diffusion ∝ 1/√M
r₁/r₂ = √(M₂/M₁)

The following are examples of implication of Graham’s law:

Light gases diffuse faster than heavy gases.
Ammonia (NH₃) diffuses faster than hydrogen chloride (HCl) because NH₃ has a lower molar mass.

Diffusion Demonstration (NH₃ and HCl)

When cotton wool soaked in ammonia solution and cotton wool soaked in concentrated HCl are placed at opposite ends of a glass tube, the gases diffuse and form a white ring of ammonium chloride (NH₄Cl).

The following are key observations and explanations:

A white ring forms inside the tube.
The ring forms nearer the HCl end because NH₃ diffuses faster.
The product formed is ammonium chloride (NH₄Cl).

Avogadro’s Law

Avogadro’s law states that equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules.

The following are important points from Avogadro’s law:

At constant temperature and pressure, volume is directly proportional to the number of moles.
V ∝ n
V/n = constant

Avogadro’s Number and the Mole Concept

A mole is the amount of substance that contains Avogadro’s number of particles. Avogadro’s number is 6.02 × 10²³ particles per mole.

The following are key relationships for the mole concept:

Number of particles (N) = n × N_A
n = N/N_A
N_A = 6.02 × 10²³ mol^-1

Molar Volume of Gases

Molar volume is the volume occupied by one mole of a gas at a stated temperature and pressure. At STP, one mole of an ideal gas occupies 22.4 dm³.

The following are key facts about molar volume:

At STP, 1 mole of gas occupies 22.4 dm³ (22.4 L).
At the same temperature and pressure, gas volume depends on number of moles.
Volume can be calculated using V = n × molar volume (when conditions match the stated molar volume).

Gay-Lussac’s Law of Combining Volumes

Gay-Lussac’s law states that volumes of reacting gases and gaseous products, measured at the same temperature and pressure, are in simple whole number ratios.

The following are examples of simple volume ratios in gaseous reactions:

2 volumes of H₂ react with 1 volume of O₂ to form 2 volumes of H₂O (steam) at the same conditions.
1 volume of N₂ reacts with 3 volumes of H₂ to form 2 volumes of NH₃ at the same conditions.

Ideal Gas Equation and Calculations (PV = nRT)

The ideal gas equation relates pressure, volume, and temperature of a gas to the amount in moles.

The following are key points for using PV = nRT:

Equation: PV = nRT
P = pressure, V = volume, n = number of moles, T = temperature in Kelvin
Temperature conversion: K = °C + 273
Rearrange the equation to find the unknown (P, V, n, or T).

The following are steps for solving gas law calculations:

Write the given values and the unknown.
Convert temperature to Kelvin where needed.
Ensure units are consistent before substitution.
Substitute correctly and solve.
State the final answer with unit.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids, Problem Solving

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher asks pupils to describe how a smell spreads in a room and links the idea to diffusion and gas behaviour under different conditions.
Pupils’ Activity: Pupils respond with examples and state what they notice about gases spreading and occupying space.
Learning Point: Gases spread by diffusion and can be studied using gas laws.

Step 2: Demonstration of Diffusion

Time: 8 minutes
Teaching Skill: Demonstration
Teacher’s Activity: The teacher demonstrates diffusion using cotton wool soaked in ammonia solution and concentrated HCl placed at opposite ends of a glass tube/vessel, then guides pupils to observe the formation and position of the white ring.
Pupils’ Activity: Pupils observe carefully and describe the result, noting where the white ring forms.
Learning Point: Diffusion occurs in gases, and lighter gases diffuse faster than heavier gases.

Step 3: Explanation of Graham’s Law

Time: 6 minutes
Teaching Skill: Explanation
Teacher’s Activity: The teacher defines diffusion, states Graham’s law, writes the relationship and equation on the board, and connects it to the observed position of the NH₄Cl ring.
Pupils’ Activity: Pupils state Graham’s law in words, copy the equations, and explain why the ring forms nearer the HCl end.
Learning Point: Graham’s law explains that diffusion rate is related to molar mass.

Step 4: Avogadro’s Law and Mole Concept

Time: 7 minutes
Teaching Skill: Explanation
Teacher’s Activity: The teacher states Avogadro’s law, explains the mole concept and Avogadro’s number, and writes the key relationships for converting between moles and number of particles.
Pupils’ Activity: Pupils relate mole concept to Avogadro’s number, copy the relationships, and answer short oral questions on n and N.
Learning Point: Avogadro’s law links gas volume to number of moles, and Avogadro’s number links moles to particles.

Step 5: Molar Volume and Volume Relations in Gaseous Reactions

Time: 6 minutes
Teaching Skill: Explanation/Illustration
Teacher’s Activity: The teacher explains molar volume at STP and states Gay-Lussac’s law of combining volumes with examples of simple whole number ratios in gaseous reactions.
Pupils’ Activity: Pupils state the molar volume at STP and interpret simple volume ratios from given reaction examples.
Learning Point: Gas volumes can be related to moles and to reaction ratios at the same temperature and pressure.

Step 6: PV = nRT and Worked Examples

Time: 8 minutes
Teaching Skill: Problem Solving
Teacher’s Activity: The teacher explains PV = nRT, emphasizes Kelvin conversion and unit consistency, and solves sample calculation questions on the board, then gives one short practice question.
Pupils’ Activity: Pupils copy the formula, solve the practice question using PV = nRT, and show steps clearly.
Learning Point: The ideal gas equation is used to solve calculation problems involving gases.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

Define diffusion and mention two characteristics of diffusion in gases.
State Graham’s law of diffusion and write its equation.
State Avogadro’s law and state the value of Avogadro’s number.
Use PV = nRT to calculate the number of moles when P = 100 kPa, V = 2.0 dm³, T = 300 K, and R is given.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 5 minutes
Teaching Skill: Reinforcement/Summary
Teacher’s Activity: The teacher summarizes diffusion, Graham’s law, Avogadro’s law, molar volume, and PV = nRT, corrects mistakes on units and Kelvin conversion, and gives a short assignment on related calculations.
Pupils’ Activity: Pupils state the main points, correct their notes, and copy the assignment.
Learning Point: Gas behaviour can be explained by diffusion laws and calculated using mole concepts and the ideal gas equation.

Lesson Keywords

Diffusion – movement of gas particles from higher concentration to lower concentration until evenly spread.
Graham’s Law – diffusion rate is inversely proportional to the square root of molar mass.
Avogadro’s Law – equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
Avogadro’s Number – 6.02 × 10²³ particles in one mole.
Mole – amount of substance containing 6.02 × 10²³ particles.
Molar Volume – volume occupied by one mole of a gas at stated conditions (22.4 dm³ at STP).
PV = nRT – ideal gas equation relating pressure, volume, moles, and temperature.

Differentiation

Pupils who need support will receive a guided template for Kelvin conversion and formula rearrangement, while faster learners will solve extra questions involving combining molar volume with PV = nRT and comparing diffusion rates using molar masses.

Note for teachers using this lesson plan

Ensure safety by handling concentrated HCl and ammonia solution carefully and using proper ventilation. Emphasize that temperature must be in Kelvin for PV = nRT, and insist on correct units and clear working steps in all calculations.