Empirical and Molecular Formulae, Separation of Mixtures for SS 1

Class: SS 1
Term: Second Term
Week: 2
Age: 15 years
Duration: 60 minutes
Subject: Chemistry
Curriculum Theme: The chemical world
Focal competence: Using symbols, formulae and equations to express the composition and interactions between elements and compounds
Key competencies/values: Creativity and Innovation; Innovation
Skills:

• Percentage composition of
• elements in a compound. empirical and molecular
• formulae of compounds
• Handling (setting up) of apparatus for the separation of given mixtures

Previous Lesson: Elements, Compounds, Mixtures and Common Elements
Topic: Elements, Compounds And Mixtures: Empirical And Molecular Formulae Of Simple Compounds
Subject Matter: Empirical and molecular formulae of simple compounds, Separation of mixtures into constituents

Specific Objectives

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

Cognitive Domain

• State the differences between elements, compounds and mixtures.
• Determine the percentage composition of elements in a compound.
• Determine the empirical and molecular formulae of simple compounds.
• Write the symbols for the first 20 elements of the Periodic Table and other common elements.

Psychomotor Domain

• Write the chemical formula of simple compounds.
• Separate mixtures into their constituent elements.
• Set up apparatus for the separation of given mixtures.

Reference Materials

The following resources were used in planning this lesson:

• 2025 New Revised Senior Secondary Education Curriculum (SSEC)
• Relevant State Unified Scheme of Work
• A suitable Chemistry textbook for SS 1
• The HeadTeacher Scheme of work For The New Revised Senior Secondary Education Curriculum (SSEC)

Instructional Materials

The teacher will teach this lesson with the aid of:

• Periodic Table of elements
• Atomic structure models
• Cardboard papers
• Tooth picks
• Paper glue
• Samples of elements (e.g., copper wire, iron filings)
• Samples of compounds (e.g., salt, sugar, water)
• Samples of mixtures (e.g., sand and salt, oil and water, ink)
• Filtration apparatus (funnel, filter paper, beaker)
• Evaporating dish, Bunsen burner, tripod stand, gauze mat
• Distillation apparatus
• Magnets

Rationale for the Lesson

This lesson provides foundational knowledge in chemistry by clarifying the distinctions between elements, compounds, and mixtures, which are the basic building blocks of matter. Understanding empirical and molecular formulae is essential for quantitative chemistry, enabling students to determine the composition of substances. Practical skills in separating mixtures are crucial for everyday life and laboratory work, fostering an appreciation for chemical principles and their applications.

Prerequisite/Previous Knowledge

Students should have a basic understanding of matter, atoms, molecules, and the concept of chemical combination from their Junior Secondary School (JSS) science classes.

Lesson Content/Board Summary

Elements, Compounds And Mixtures: Empirical And Molecular Formulae Of Simple Compounds

Elements, Compounds, and Mixtures

Elements

An element is a pure substance that cannot be broken down into simpler substances by ordinary chemical means. Elements are composed of only one type of atom. Each element has a unique atomic number and is represented by a chemical symbol.

Characteristics of Elements:

1. They are pure substances.
2. They cannot be decomposed into simpler substances by chemical reactions.
3. They are the basic building blocks of all matter.
4. Each element has unique physical and chemical properties.

Examples of Elements and their Symbols:

1. Hydrogen (H)
2. Helium (He)
3. Lithium (Li)
4. Beryllium (Be)
5. Boron (B)
6. Carbon (C)
7. Nitrogen (N)
8. Oxygen (O)
9. Fluorine (F)
10. Neon (Ne)
11. Sodium (Na)
12. Magnesium (Mg)
13. Aluminium (Al)
14. Silicon (Si)
15. Phosphorus (P)
16. Sulphur (S)
17. Chlorine (Cl)
18. Argon (Ar)
19. Potassium (K)
20. Calcium (Ca)
21. Iron (Fe)
22. Copper (Cu)
23. Zinc (Zn)
24. Silver (Ag)
25. Gold (Au)
26. Lead (Pb)
27. Mercury (Hg)

Compounds

A compound is a pure substance formed when two or more different elements combine chemically in a fixed ratio by mass. The properties of a compound are entirely different from those of its constituent elements. Compounds can only be broken down into simpler substances by chemical reactions.

Characteristics of Compounds:

1. They are pure substances.
2. They are formed by the chemical combination of two or more elements.
3. The elements are combined in a fixed proportion by mass.
4. The properties of a compound are different from those of its constituent elements.
5. They have a fixed melting point and boiling point.
6. They can only be separated into their constituent elements by chemical methods.

Examples of Compounds:

1. Water (H₂O)
2. Carbon Dioxide (CO₂)
3. Sodium Chloride (NaCl)
4. Sulphuric Acid (H₂SO₄)
5. Glucose (C₆H₁₂O₆)

Mixtures

A mixture is a substance containing two or more elements or compounds that are physically combined but not chemically bonded. The components of a mixture retain their individual properties and can be separated by physical means.

Characteristics of Mixtures:

1. They contain two or more substances physically combined.
2. The components are not chemically bonded.
3. The components retain their individual properties.
4. The components can be present in any proportion.
5. They do not have fixed melting or boiling points.
6. They can be separated by physical methods.

Types of Mixtures:

1. Homogeneous Mixtures: Have a uniform composition throughout (e.g., saltwater, air).
2. Heterogeneous Mixtures: Have a non-uniform composition; components are visibly distinct (e.g., sand and water, oil and water).

Examples of Mixtures:

1. Air (mixture of nitrogen, oxygen, argon, etc.)
2. Saltwater (mixture of salt and water)
3. Sand and iron filings
4. Crude oil
5. Blood

Differences between Elements, Compounds, and Mixtures

Feature	Element	Compound	Mixture
Composition	One type of atom	Two or more different elements chemically combined in a fixed ratio	Two or more substances physically combined in any ratio
Bonding	No chemical bonds between different types of atoms (only one type)	Chemical bonds between constituent atoms	No chemical bonds between components
Properties	Retains its own unique properties	Properties are entirely different from constituent elements	Components retain their individual properties
Separation	Cannot be broken down by chemical means	Can only be separated by chemical means	Can be separated by physical means
Energy Change	No energy change during formation (it’s fundamental)	Energy is usually absorbed or released during formation	No significant energy change during formation
Formula/Symbol	Represented by a symbol (e.g., O, H)	Represented by a chemical formula (e.g., H₂O, CO₂)	No fixed chemical formula

Empirical and Molecular Formulae

Empirical Formula

The empirical formula of a compound represents the simplest whole-number ratio of the atoms of each element present in the compound. It gives the relative number of atoms of each element.

Example: The empirical formula of glucose is CH₂O, even though its molecular formula is C₆H₁₂O₆. The ratio 6:12:6 simplifies to 1:2:1.

Molecular Formula

The molecular formula of a compound shows the actual number of atoms of each element present in one molecule of the compound. It represents the true composition of the molecule.

Example: The molecular formula of glucose is C₆H₁₂O₆, indicating that one molecule of glucose contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.

Relationship between Empirical and Molecular Formulae

The molecular formula is a simple whole-number multiple of the empirical formula.

(text{Molecular Formula} = n times text{Empirical Formula})

Where (n) is a whole number (1, 2, 3, …). The value of (n) can be determined using the relationship:

(n = frac{text{Molar Mass of Compound}}{text{Empirical Formula Mass}})

Determination of Percentage Composition by Mass

The percentage composition by mass of an element in a compound is the mass of that element present in 100 parts by mass of the compound.

Formula

(% text{ by mass of an element} = frac{text{Mass of element in one mole of compound}}{text{Molar mass of compound}} times 100%)

Example 1

Question: Calculate the percentage composition by mass of hydrogen and oxygen in water (H₂O). (Given: H = 1, O = 16)

Solution:

Step 1: Calculate the molar mass of H₂O.

(text{Molar mass of H₂O} = (2 times 1) + 16 = 2 + 16 = 18 text{ g/mol})

Step 2: Calculate the percentage of Hydrogen.

(% text{ H} = frac{text{Mass of H in H₂O}}{text{Molar mass of H₂O}} times 100% = frac{2 times 1}{18} times 100% = frac{2}{18} times 100% = 11.11%)

Step 3: Calculate the percentage of Oxygen.

(% text{ O} = frac{text{Mass of O in H₂O}}{text{Molar mass of H₂O}} times 100% = frac{16}{18} times 100% = 88.89%)

Answer: Hydrogen = 11.11%, Oxygen = 88.89%

Determination of Empirical Formula

The empirical formula can be determined from the percentage composition by mass of the elements in a compound.

Steps:

1. Assume 100 g of the compound, so the percentage by mass becomes the mass in grams.
2. Convert the mass of each element to moles by dividing by its atomic mass.
3. Divide each mole value by the smallest number of moles obtained to get the simplest mole ratio.
4. If the ratios are not whole numbers, multiply all ratios by the smallest whole number that converts them into whole numbers.
5. Write the empirical formula using these whole-number ratios as subscripts.

Example 1

Question: A compound contains 40.0% Carbon, 6.7% Hydrogen, and 53.3% Oxygen by mass. Determine its empirical formula. (Given: C = 12, H = 1, O = 16)

Solution:

Step 1: Assume 100 g of the compound.

1. Mass of Carbon = 40.0 g
2. Mass of Hydrogen = 6.7 g
3. Mass of Oxygen = 53.3 g

Step 2: Convert mass to moles.

1. Moles of Carbon = (frac{40.0 text{ g}}{12 text{ g/mol}} = 3.33 text{ mol})
2. Moles of Hydrogen = (frac{6.7 text{ g}}{1 text{ g/mol}} = 6.7 text{ mol})
3. Moles of Oxygen = (frac{53.3 text{ g}}{16 text{ g/mol}} = 3.33 text{ mol})

Step 3: Divide by the smallest number of moles (3.33).

1. Ratio for Carbon = (frac{3.33}{3.33} = 1)
2. Ratio for Hydrogen = (frac{6.7}{3.33} approx 2)
3. Ratio for Oxygen = (frac{3.33}{3.33} = 1)

Step 4: Write the empirical formula.

Answer: The empirical formula is CH₂O.

Determination of Molecular Formula

The molecular formula can be determined if the empirical formula and the molar mass of the compound are known.

Steps:

1. Determine the empirical formula of the compound.
2. Calculate the empirical formula mass (EFM).
3. Calculate the value of (n) using the formula: (n = frac{text{Molar Mass}}{text{Empirical Formula Mass}}).
4. Multiply the subscripts of the empirical formula by (n) to get the molecular formula.

Example 1

Question: A compound has an empirical formula of CH₂O and a molar mass of 180 g/mol. Determine its molecular formula. (Given: C = 12, H = 1, O = 16)

Solution:

Step 1: Empirical formula is CH₂O.

Step 2: Calculate the empirical formula mass (EFM).

(text{EFM of CH₂O} = 12 + (2 times 1) + 16 = 12 + 2 + 16 = 30 text{ g/mol})

Step 3: Calculate the value of (n).

(n = frac{text{Molar Mass}}{text{Empirical Formula Mass}} = frac{180 text{ g/mol}}{30 text{ g/mol}} = 6)

Step 4: Multiply the subscripts of the empirical formula by (n).

(text{Molecular Formula} = (text{CH₂O})_6 = text{C}_{1 times 6}text{H}_{2 times 6}text{O}_{1 times 6} = text{C}_6text{H}_{12}text{O}_6)

Answer: The molecular formula is C₆H₁₂O₆.

Separation of Mixtures

Mixtures can be separated into their constituent components using various physical methods, which exploit differences in the physical properties of the components.

Methods of Separating Mixtures:

1. Decantation: Used to separate immiscible liquids (liquids that do not mix, e.g., oil and water) or a liquid from a solid sediment by carefully pouring off the liquid.
2. Filtration: Used to separate an insoluble solid from a liquid. The mixture is passed through a filter medium (like filter paper) that allows the liquid (filtrate) to pass through but retains the solid (residue).
   1. Example: Separating sand from water.
3. Evaporation/Crystallisation: Used to separate a soluble solid from its solvent. The solvent is heated and allowed to evaporate, leaving the solid behind. Crystallisation involves slow evaporation to obtain pure crystals.
   1. Example: Separating salt from water.
4. Distillation: Used to separate a liquid from a soluble solid or to separate two miscible liquids with different boiling points. The liquid with the lower boiling point evaporates first, condenses, and is collected.
   1. Simple Distillation: For separating a volatile liquid from a non-volatile solute (e.g., pure water from saltwater).
   2. Fractional Distillation: For separating two or more miscible liquids with close but different boiling points (e.g., ethanol and water, crude oil components).
5. Sublimation: Used to separate a solid that sublimes (changes directly from solid to gas without passing through the liquid state) from a solid that does not.
   1. Example: Separating iodine from salt, or ammonium chloride from sodium chloride.
6. Chromatography: A technique used to separate components of a mixture based on their differential movement through a stationary phase (e.g., paper or silica gel) by a mobile phase (solvent).
   1. Example: Separating different coloured dyes in ink.
7. Magnetism: Used to separate magnetic substances from non-magnetic substances. A magnet is passed over the mixture to attract the magnetic component.
   1. Example: Separating iron filings from sand.
8. Sieving: Used to separate solid particles of different sizes by passing them through a sieve with appropriate mesh size.
   1. Example: Separating garri from chaff, or sand from gravel.

Teaching Methods/Instructional Techniques

Discussion, Demonstration, Guided Practice, Question and Answer, Explanation, Group Work, Practical Activity, Problem Solving.

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Recalling/Engaging

Teacher’s Activity: The teacher greets the students and reviews previous knowledge on matter, atoms, and basic classification of substances as elements, compounds, or mixtures, asking questions like “What is an atom?” and “Can you give an example of a pure substance?”.

Pupils’ Activity: Pupils respond to questions and share their prior knowledge.

Learning Point: Review of basic substances

Step 2: Elements, Compounds, and Mixtures

Time: 10 minutes

Teaching Skill: Explaining/Illustrating

Teacher’s Activity: The teacher explains the definitions and characteristics of elements, compounds, and mixtures, using the Periodic Table and atomic models to illustrate. The teacher guides students to use cardboard papers and toothpicks in groups to build simple atomic models of compounds like H₂O or CO₂ and write their formulae (Activity 1).

Pupils’ Activity: Pupils listen, observe the models, participate in building models, and write down definitions and examples.

Learning Point: Substance classification and models

Step 3: Percentage Composition

Time: 10 minutes

Teaching Skill: Explaining/Calculating

Teacher’s Activity: The teacher introduces the concept of percentage composition by mass, explains the formula, and works through an example on the board to demonstrate its calculation.

Pupils’ Activity: Pupils listen, copy the formula, and follow the steps of the worked example, asking questions for clarification.

Learning Point: Calculating percentage composition

Step 4: Empirical Formula Determination

Time: 10 minutes

Teaching Skill: Problem Solving/Guiding

Teacher’s Activity: The teacher explains the steps for determining the empirical formula from percentage composition and works through a detailed example on the board, guiding students through each step.

Pupils’ Activity: Pupils pay attention to the steps, copy the example, and attempt to solve similar problems if given a quick practice question.

Learning Point: Determining empirical formula

Step 5: Molecular Formula Determination

Time: 5 minutes

Teaching Skill: Connecting/Applying

Teacher’s Activity: The teacher explains the relationship between empirical and molecular formulae and demonstrates how to determine the molecular formula using the empirical formula and molar mass, working through an example.

Pupils’ Activity: Pupils listen, understand the relationship, and copy the example, relating it to the empirical formula concept.

Learning Point: Calculating molecular formula

Step 6: Separation of Mixtures

Time: 10 minutes

Teaching Skill: Demonstrating/Facilitating

Teacher’s Activity: The teacher introduces various methods for separating mixtures, explaining the principle behind each. The teacher then guides students in small groups to separate different types of mixtures using appropriate techniques (e.g., filtration of sand and water, magnetic separation of iron filings and sand) (Activity 2).

Pupils’ Activity: Pupils observe demonstrations, participate in group practical activities, and discuss the separation methods.

Learning Point: Methods for separating mixtures

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

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

1. State two differences between an element and a compound.
2. Calculate the percentage by mass of Carbon in CO₂. (C=12, O=16)
3. A compound has an empirical formula of CH₂ and a molar mass of 42 g/mol. What is its molecular formula?
4. Mention two methods used to separate a mixture of sand and salt.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Understanding formulae and separation

Step 8: Note-Taking

Time: 10 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on elements, compounds, mixtures, empirical/molecular formulae, and separation methods into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Recording lesson notes

Step 9: Conclusion

Time: 5 minutes

Teaching Skill: Summarising/Reinforcing

Teacher’s Activity: The teacher briefly summarises the key concepts covered: the distinctions between elements, compounds, and mixtures, the importance of empirical and molecular formulae in representing compounds, and the practical applications of mixture separation techniques. The teacher encourages students to review their notes.

Pupils’ Activity: Pupils listen attentively and ask any final questions.

Learning Point: Consolidation of key concepts

Continuous Assessment/Further Study

Type: Homework

Instruction: Answer the following questions in your notebook.

1. Differentiate between a homogeneous and a heterogeneous mixture, giving two examples for each.
2. A compound contains 2.2% Hydrogen, 26.7% Carbon, and 71.1% Oxygen. Determine its empirical formula. (H=1, C=12, O=16)
3. If the compound in question 2 has a molar mass of 90 g/mol, determine its molecular formula.
4. Describe the process of separating a mixture of common salt and iron filings.
5. Write the chemical symbols for Sodium, Chlorine, Potassium, and Magnesium.

Lesson Keywords

• Element – A pure substance that cannot be broken down into simpler substances.
• Compound – A pure substance formed by chemical combination of two or more elements in a fixed ratio.
• Mixture – A substance containing two or more components physically combined.
• Empirical Formula – The simplest whole-number ratio of atoms in a compound.
• Molecular Formula – The actual number of atoms of each element in a molecule.
• Percentage Composition – The mass of an element in 100 parts by mass of a compound.
• Decantation – Separation of immiscible liquids or liquid from sediment.
• Filtration – Separation of insoluble solid from liquid.
• Distillation – Separation based on differences in boiling points.
• Sublimation – Solid changing directly to gas.
• Chromatography – Separation based on differential movement through a medium.

Differentiation

Support: Provide simpler examples for calculations and offer pre-drawn diagrams for separation techniques. Pair struggling students with more capable peers for group activities. Focus on understanding definitions and basic differences.

Extension: Challenge advanced students to research and present on more complex separation techniques (e.g., fractional distillation of crude oil) or to determine empirical formulae from experimental data involving mass changes.

Suggested Lesson Videos

For further understanding, search on YouTube for:

• “Elements Compounds Mixtures SS1 Chemistry”
• “Empirical and Molecular Formulae calculations Chemistry”
• “Separation of Mixtures techniques SS1 Chemistry”