KASEMPA DAY SECONDARY
Grade/Class: Grade 11 Number of Pupils in Class: …………. Date: 2026-05-18 Duration: 80 minutes
Name of Teacher: MR MASUMBA D
Subject: Chemistry Topic: THE MOLE CONCEPT Subtopic: Relative masses Rationale: This is lesson number one in the THE MOLE CONCEPT study series. Relative masses are important because they help learners in Zambia to interpret chemical formulae correctly, compare masses of atoms and molecules, and perform calculations needed in further Chemistry learning such as moles, equations, and quantitative analysis. The lesson will use a Learner-Centered Approach so that learners actively build meaning from prior knowledge and guided examples. Discussion Method, Demonstration Method, Practical Work Method, and Question & Answer Method will be used to help learners describe Relative Atomic Mass and relative molecular mass, and accurately calculate relative formula mass through guided questioning, use of charts/diagrams, think-pair-share, and group work. Specific Outcome(s): By the end of the lesson, learners should be able to: • Describe Relative Atomic Mass and relative molecular mass. • Calculate the relative formula mass of a compound Prerequisite Knowledge: • Structure of atoms in terms of protons, neutrons, and electrons • Meaning and use of chemical symbols and formulae such as H2O, CO2, NaCl, and CaCO3 • Reading simple information from the periodic table, including atomic masses • Addition and multiplication of whole numbers and decimals in calculations • Difference between an atom, a molecule, and a compound References: • Chemistry Pupil's Textbook, pg. 42–47 • New Secondary Chemistry for Zambia Grade 11, pg. 55–60 • Zambia Senior Secondary School Chemistry Syllabus, pg. 18–19 Knowledge: RAM as relative mass of an element's isotopes as compared to carbon-12; RMM as relative mass of a molecule as compared to carbon-12; As the sum of the relative atomic masses of all the atoms in the compound Skills: Mathematical calculations; Formula manipulation; Data interpretation Values: Accuracy; Logical reasoning; Systematic problem-solving Teaching / Learning Aids: • Manila chart 1: Table titled “Selected Elements and Relative Atomic Masses” with 3 columns labelled Element, Symbol, Relative Atomic Mass and 8 rows containing Hydrogen—H—1, Carbon—C—12, Nitrogen—N—14, Oxygen—O—16, Sodium—Na—23, Magnesium—Mg—24, Sulfur—S—32, Chlorine—Cl—35.5 • Manila chart 2: Definition chart with two boxed sections labelled Relative Atomic Mass (Ar) and Relative Molecular Mass (Mr); under each box is a written definition comparing mass to 112 of the mass of a carbon-12 atom; bottom section has examples H2O, CO2, NH3 with spaces left for calculation • Manila chart 3: Worked-example chart titled “How to Calculate Relative Formula Mass” arranged in 4 numbered steps: 1. Write the formula, 2. Identify each element and number of atoms, 3. Insert relative atomic masses, 4. Add total; examples shown are NaCl, MgO, and CaCO3 with labelled atom counts • Manila chart 4: Practice table with 4 columns labelled Compound, Atoms Present, Relative Atomic Masses Used, Relative Formula Mass and 4 rows for H2O, CO2, NH3, H2SO4 left partly blank for learner completion • Manila chart 5: Summary chart titled “Key Rules for Relative Mass Calculations” with five numbered statements: use correct chemical formula, count every atom, multiply where a symbol has a subscript, add all values, write the final answer without unit Alternative Materials: • Whiteboard drawings • Projected slides • Printed handouts METHODOLOGIES, STRATEGIES AND APPROACHES: Approach: Learner-Centered Approach Method: • Question & Answer Method — Introduction, Step 4 • Demonstration Method — Development Step 1 • Discussion Method — Step 2 • Practical Work Method — Step 3 Strategy: • Brainstorming — Introduction • Use of Charts/Diagrams — Development Step 1, Step 2, Step 3 • Guided Questioning — Development Step 1, Step 4 • Think-Pair-Share — Step 2 • Group Work — Step 3 Lesson Implementation: [TABLE_START] Stage|Teaching Methods|Teacher's Activities|Learner's Activities|Learning Points Introduction - 10 min|Question & Answer Method|Teacher greets learners and displays Manila chart 1: Table titled “Selected Elements and Relative Atomic Masses.” Teacher asks: “What is an atom?” “What does the subscript in H2O show?” and “Using the chart, what are the relative atomic masses of H, O, C, and Cl?” Then names the lesson subtopic as Relative masses.|Learners answer: “An atom is the smallest particle of an element.” “The subscript in H2O shows there are 2 hydrogen atoms.” “The relative atomic masses are H = 1, O = 16, C = 12, and Cl = 35.5.” Learners state that today’s subtopic is Relative masses.|Atom, molecule, and chemical formula recall. Relative atomic masses are values used to compare masses of atoms, for example H = 1, O = 16, C = 12, Cl = 35.5. Development Step 1 - 20 min|Demonstration Method|Teacher displays Manila chart 2 and points to the two boxed sections labelled Relative Atomic Mass (Ar) and Relative Molecular Mass (Mr). Teacher explains that Relative Atomic Mass is the mass of an atom of an element compared with 1/12 of the mass of a carbon-12 atom, and Relative Molecular Mass is the mass of a molecule compared with 1/12 of the mass of a carbon-12 atom. Teacher then uses all the examples shown on the chart to demonstrate H2O: Mr = (2 × 1) + 16 = 18, CO2: Mr = 12 + (2 × 16) = 44, and NH3: Mr = 14 + (3 × 1) = 17.|Learners study Manila chart 2, copy the two definitions, and note that both are compared with 1/12 of the mass of a carbon-12 atom. Learners record all the chart examples: H2O has Mr = 18, CO2 has Mr = 44, and NH3 has Mr = 17.|1. Relative Atomic Mass, Ar, is the relative mass of an element’s isotopes compared with 112 of the mass of a carbon-12 atom. 2. Relative Molecular Mass, Mr, is the relative mass of a molecule compared with 112 of the mass of a carbon-12 atom. 3. Example: H2O, Mr = (2 × 1) + 16 = 18. 4. Example: CO2, Mr = 12 + (2 × 16) = 44. Step 2 - 18 min|Discussion Method|Teacher displays Manila chart 4 with the columns Compound, Atoms Present, Relative Atomic Masses Used, and Relative Formula Mass. Teacher asks: “What atoms are present in H2O, CO2, NH3, and H2SO4?” “How many atoms of each element appear in H2O, CO2, NH3, and H2SO4?” and “What rule should we follow before adding the values in the Relative Formula Mass column?” Teacher manages pair discussion and class sharing without giving full solutions.|Learners discuss in pairs and share in order: H2O contains H and O and has 2 H and 1 O; CO2 contains C and O and has 1 C and 2 O; NH3 contains N and H and has 1 N and 3 H; H2SO4 contains H, S, and O and has 2 H, 1 S, and 4 O. Learners then state that before adding values in the Relative Formula Mass column, they must count every atom and multiply where a symbol has a subscript.|Relative formula mass is found by summing the relative atomic masses of all atoms in a compound. Example of correct learner output: NH3 contains 1 N and 3 H, so the values needed are 14 and 1 used three times. Step 3 - 22 min|Practical Work Method|Teacher displays Manila chart 3 and Manila chart 1. Teacher guides groups: “Using the 4 steps on Manila chart 3 and the values on Manila chart 1, calculate the relative formula mass of NaCl and MgO. For each compound, write the atoms present, the relative atomic masses used, and the final total. Follow all four steps: write the formula, identify each element and number of atoms, insert relative atomic masses, and add the total.” Teacher circulates and prompts groups to follow the four steps without giving answers.|Learners work in groups using Manila chart 3 and Manila chart 1. For NaCl, they write the formula, identify 1 Na and 1 Cl, use Na = 23 and Cl = 35.5, and add to get 58.5. For MgO, they write the formula, identify 1 Mg and 1 O, use Mg = 24 and O = 16, and add to get 40.|Task: Calculate the relative formula mass of NaCl, MgO, and CaCO3. Framework: 1. Write the formula. 2. Identify each element and number of atoms. 3. Insert relative atomic masses. 4. Add the total. Key values: Na = 23, Cl = 35.5, Mg = 24, O = 16, C = 12, Ca = 40. Step 4 - 10 min|Question & Answer Method|Teacher asks: “What is Relative Atomic Mass?” “What is Relative Molecular Mass?” and “What are the relative formula masses of NaCl, MgO, and CaCO3?” Teacher confirms correct answers, corrects any counting errors, and gives homework: “Calculate the relative formula mass of NH3, CO2, and H2SO4.”|Learners answer: “Relative Atomic Mass is the relative mass of an element’s isotopes compared with 112 of the mass of a carbon-12 atom.” “Relative Molecular Mass is the relative mass of a molecule compared with 112 of the mass of a carbon-12 atom.” “NaCl = 23 + 35.5 = 58.5, MgO = 24 + 16 = 40, CaCO3 = 40 + 12 + (3 × 16) = 100.” Learners record the homework task.|Summary: Ar describes atoms, Mr describes molecules, and relative formula mass is the sum of the relative atomic masses of all atoms in the compound. Worked solutions: NaCl = 58.5, MgO = 40, CaCO3 = 100. Homework: Calculate the relative formula mass of NH3, CO2, and H2SO4. 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