SSS 1 Chemistry Lesson Plan for 2nd term week 3: Percentage Composition and Empirical Formula

 

Term: 2nd Term

Week: 3

Class: SSS 1 (Senior Secondary School 1)

Subject: Chemistry

Topic: Percentage Composition and Empirical Formula

Duration: 80 Minutes (Double Period)

I. Lesson Objectives

By the end of this double-period lesson, SSS 1 students should be able to:

1. Define percentage composition and empirical formula.

2. Calculate the percentage composition by mass of individual elements within a given compound.

3. Determine the empirical formula of a compound using mass or percentage composition analytical data.

II. Cognitive Entry Behavior (Prior Knowledge)

Students are expected to already know:

• How to write basic chemical formulas.

• How to calculate the Relative Molecular Mass (RMM) or Molar Mass of compounds using Relative Atomic Masses (Ar).

• The fundamental relationship between mass, molar mass, and moles (n = m/M).

III. Instructional Materials

• Whiteboard, markers, and eraser.

• Scientific calculators (essential for every student).

• Periodic Table of Elements (or written Relative Atomic Masses on the board).

• A pre-drawn "Empirical Formula Table Method" chart on a poster sheet or written clearly on the board.

IV. Lesson Delivery Method & Step-by-Step Procedure

1. Introduction / The Hook (10 Minutes)

• Teacher's Activity: Walk into the class and write a chemical formula on the board, for example, H2O. Ask the class: "If I have 18 grams of water, does it mean I have equal masses of Hydrogen and Oxygen inside it?" Allow the students to think and answer.

• Explanation: Lead them to realize that because Oxygen atoms are much heavier than Hydrogen atoms, the mass is not shared equally. Explain that in chemical manufacturing, pharmaceutical production, and mineral mining, analytical chemists must calculate exactly what percentage of a compound's total mass is made up of a specific element. This is called Percentage Composition. Introduce the Empirical Formula as the simplest chemical footprint we get when analyzing a new substance in a lab.

2. Period 1: Percentage Composition of Compounds (30 Minutes)

A. Definition & Mathematical Formula

Explain that percentage composition is the percentage by mass of each element present in a chemical compound.

Write the core mathematical formula on the board:

X100/1

B. Step-by-Step Worked Example

Example Problem: Calculate the percentage composition of each element in Sodium Trioxocarbonate(IV), Na2CO3.

(Given Relative Atomic Masses: Na = 23, C = 12, O = 16)

• Step 1: Calculate the Molar Mass of the compound (Na2CO3).

  Molar Mass = (2 x 23) + (1 x 12) + ( 3x16 )
  Molar Mass = 46 + 12 + 48 = 106g/mol

• Step 2: Calculate the percentage of Sodium (Na).

  Percentage of  Na = 46/106 x 100% = 43.4

• Step 3: Calculate the percentage of Carbon (C).

  Percentage of C = 12/106 x 100% = 11.3

• Step 4: Calculate the percentage of Oxygen (O).

  Percentage of O = 48 x 106 x100% = 45.3%

• Step 5: Verify the sum.

  Show the students that the sum should equal approximately 100%:

  43.4% + 11.3% + 45.3% = 100%

3. Period 2: Empirical Formula Calculations (30 Minutes)

A. Definition

Explain that the Empirical Formula represents the simplest, lowest whole-number ratio of the atoms of different elements present in a compound. For example, while glucose has a molecular formula of C6H12O6, its simplest or empirical formula is CH2O.

B. The 4-Step Empirical Table Method

Teach students to set up their empirical calculations using a standard tabular layout. This eliminates confusion and guarantees structured thinking.

Step	Operation Name	Mathematical Action
Step 1	Write Mass / %	Write down the given mass (g) or percentage (%) of each element.
Step 2	Calculate Moles	Divide each element's mass/% by its Relative Atomic Mass (Ar).
Step 3	Divide by Smallest	Divide all the resulting mole values by the smallest mole value from Step 2.
Step 4	Find Whole Numbers	Round to the nearest whole number. (If a fraction like .5 or .33 remains, multiply all values by a common factor like 2 or 3 to clear the fraction).

C. Step-by-Step Worked Example

Example Problem: A hydrocarbon sample is analyzed and found to contain 85.7% Carbon and 14.3% Hydrogen by mass. Determine its empirical formula.

(Given Relative Atomic Masses: C = 12, H = 1)

• Tabular Calculation:

Process Step	Carbon (C)	Hydrogen (H)
1. Write Mass or %	85.7g	14.3g}
2. Divide by Ar (Calculate Moles)	85.7/12 = 7.14 moles	14.3/1  = 14.3 moles
3. Divide by the Smallest value (7.14)	7.14 /7.14 = 1	14.3/7.14 = 2.00
4. Simplest Whole Number Ratio	1	2

• Conclusion: The empirical formula of the hydrocarbon is CH2.

V. Classroom Evaluation & Practice (8 Minutes)

Write these two questions on the board for students to solve in their exercise notebooks. Walk around the classroom to offer guidance, checking for common mistakes (e.g., dividing by the diatomic mass of oxygen instead of the atomic mass).

1. Percentage Composition: Calculate the percentage of nitrogen by mass in Ammonium Tetraoxosulfate(VI), NH4)2SO4.

   (Given: N = 14, H = 1, S = 32, O = 16)

2. Empirical Formula: A sample of a compound contains 4.0 g of Calcium and 7.1 g of Chlorine. What is its empirical formula?

   (Given: Ca = 40, Cl = 35.5)

VI. Lesson Summary & Wrap-up (2 Minutes)

• Recap that Percentage Composition gives the mass ratio of elements within a compound and is calculated from the chemical formula and atomic masses.

• Recap that the Empirical Formula is the simplest chemical ratio of elements in a compound, which we calculate systematically using the 4-step table method (Mass 

•  Moles Divide by Smallest  Simplest Ratio).

VII. Homework Assignment

1. An inorganic compound is analyzed and found to have the following composition: 29.1% Sodium (Na), 40.5 Sulfur (S), and 30.4% Oxygen (O). Determine its empirical formula. (Given: Na = 23, S = 32, O = 16)

2. Calculate the percentage of water of crystallization in hydrated Copper(II) Tetraoxosulfate(VI), CuSO4  5H2O. (Given: Cu = 64, S = 32, O = 16, H = 1)