FIRST TERM E-LEARNING NOTES

SUBJECT: CHEMISTRY

CLASS: SS1

SCHEME OF WORK

WEEKLY LESSON NOTE

WEEK: 5

TOPIC: FORMULAE AND EQUATIONS

CONTENTS:

1. Chemical Formulae
2. Chemical Equations
3. Empirical and Molecular Formulae
4. Relative Molecular Mass and Percentage Composition by Mass of an Element

PERIOD 1: CHEMICAL FORMULAE

A chemical formula is a combination of two or more symbols that represents one molecule of a compound. For example, the formula for tetraoxosulphate(VI) acid is H₂SO₄. This formula indicates that in one molecule of tetraoxosulphate(VI) acid, there are two hydrogen atoms, one sulfur atom, and four oxygen atoms.

Examples of Compounds and Their Formulae:

EVALUATION

Write the chemical formulae of the following:

1. Sodium oxide, Calcium oxide, Aluminium oxide.
2. Carbon(IV) oxide, Iron(II) oxide, Copper(I) oxide.
3. Sodium tetraoxosulphate(VI), Calcium trioxonitrate(V), Magnesium trioxocarbonate(IV).

PERIOD 2: CHEMICAL EQUATIONS

Chemical reactions are represented in the form of equations that show the reactants and products involved in a given chemical reaction. For example, the reaction of aqueous hydrogen chloride and aqueous sodium hydroxide is represented by the equation:

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Balancing Chemical Equations

All chemical equations must be balanced to comply with the law of conservation of mass.

Example of Balancing the Combustion of Ammonia:

1. Reactants: NH₃ and O₂
   Products: NO and H₂O

   Unbalanced Equation:
   NH₃(g) + O₂(g) → NO(g) + H₂O(g)

2. Place a 4 in front of NH₃ and a 6 in front of H₂O:
   4NH₃(g) + O₂(g) → NO(g) + 6H₂O(g)
3. Place a 4 in front of NO to balance the nitrogen atoms:
   4NH₃(g) + O₂(g) → 4NO(g) + 6H₂O(g)
4. Finally, place a 5 in front of O₂ to balance the oxygen:
   4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g)

A balanced equation contains the same number of atoms of each element on both sides, and must also indicate the physical states of the reactants and products (aqueous, gaseous, or solid).

EVALUATION

Balance the following equations:

1. CaCO₃(s) + HCl(aq) → CaCl₂(s) + H₂O(l) + CO₂(g)
2. CO₂(g) + NaOH(aq) → Na₂CO₃(aq) + H₂O(l)

PERIOD 3: EMPIRICAL AND MOLECULAR FORMULAE

An empirical formula is the simplest formula that shows the ratio of different atoms in a compound without giving the exact number of each atom, while the molecular formula provides the exact number of atoms present in a molecule of the compound.

Example of Finding Empirical and Molecular Formulae:

1. Example Calculation: Find the empirical formula of a compound with 80% carbon and 20% hydrogen by mass.
   • Empirical Formula: CH₃
2. Example Calculation: For a compound with the following masses: C = 2.0 g, H = 0.34 g, O = 2.67 g, and a relative molecular mass of 60:
   • Calculate empirical formula: CH₂O
   • Molecular formula: C₂H₄O₂ or CH₃COOH

EVALUATION:

Calculate the empirical formula for a compound with 15.8% Al, 28.1% S, and 56.1% O.

PERIOD 4: RELATIVE MOLECULAR MASS, MOLAR MASS, AND PERCENTAGE COMPOSITION

The relative molecular mass is the sum of the relative atomic masses of all the atoms present in one molecule of the compound. It is a measure of how much heavier a mole of the substance is compared to one-twelfth the mass of a carbon-12 atom, and it has no unit.

Example Calculations:

• For NaCl: Relative molecular mass = 23 + 35.5 = 58.5
• For Ethanol (C₂H₅OH):
  Relative molecular mass = (12×2) + (1×5) + 16 = 46

Molar Mass

The molar mass is the relative molecular mass expressed in grams. For example, the molar mass of ethanol is 46 g/mol.

Percentage Composition of a Compound

To calculate the percentage composition of ethanol (C₂H₅OH):

1. Molar Mass: (12×2) + (1×5) + (16) = 46 g/mol
2. Calculate masses:
   • Mass of Carbon = 12 × 2 = 24 g
   • Mass of Hydrogen = 1 × 6 = 6 g
   • Mass of Oxygen = 16 g
3. Calculate percentages:
   • Percentage of C = (24/46) × 100 = 52.17%
   • Percentage of H = (6/46) × 100 = 13.04%
   • Percentage of O = (16/46) × 100 = 34.78%

Calculation of Chemical Formula from Percentage Composition

To find the simplest chemical formula given percentage composition:

• Example: For a compound with Na = 43.40%, C = 11.32%, O = 45.28%:
  • Moles of each element can be calculated, leading to the molar ratio and ultimately to the simplest formula.

GENERAL EVALUATION

ESSAY QUESTIONS:

1. Differentiate between valency and oxidation number.
2. Determine the empirical formula of an oxide of nitrogen containing 70% oxygen, if the relative molecular mass of the oxide is 92. Deduce its molecular formula.
3. Balance the chemical equation:
   NaOH + H₂SO₄ → Na₂SO₄ + H₂O

Additional Resources for Enhanced Understanding:

• Khan Academy – Chemical Formulae
• ChemCollective – Balancing Chemical Equations
• Royal Society of Chemistry – Empirical and Molecular Formula

These resources provide comprehensive explanations and examples that will further aid students in understanding the concepts of chemical formulae, equations, and related calculations.

EVALUATION QUESTIONS

1. The chemical formula of water is ____.
   a) H₂O
   b) H₂SO₄
   c) CO₂
   d) NaCl
2. The empirical formula represents ____.
   a) the exact number of atoms
   b) the ratio of atoms in a compound
   c) the chemical properties
   d) molecular mass
3. The molecular formula of ethanol is ____.
   a) C₂H₆
   b) CH₄
   c) C₂H₅OH
   d) CO₂
4. A chemical equation is balanced to satisfy the ____.
   a) law of gravity
   b) law of conservation of mass
   c) law of thermodynamics
   d) ideal gas law
5. In the formula NaCl, Na represents ____.
   a) sodium
   b) nitrate
   c) nitrogen
   d) neon
6. The symbol O₂ represents ____.
   a) one oxygen atom
   b) one oxygen molecule
   c) two oxygen atoms
   d) ozone
7. The simplest formula that gives the ratio of different atoms in a compound is called ____.
   a) empirical formula
   b) molecular formula
   c) valency
   d) isotopes
8. The chemical formula for calcium chloride is ____.
   a) CaCl₂
   b) CaCl
   c) Ca₂Cl
   d) Ca₂Cl₂
9. Which of the following is a diatomic molecule?
   a) H₂
   b) HCl
   c) O₂
   d) All of the above
10. A balanced chemical equation has the same number of ____ on both sides.
    a) molecules
    b) atoms
    c) ions
    d) compounds
11. The formula mass of NaCl is approximately ____.
    a) 58.5
    b) 44
    c) 30
    d) 24
12. If a compound contains 80% carbon and 20% hydrogen by mass, its empirical formula is ____.
    a) CH₄
    b) C₂H₄
    c) CH₃
    d) C₄H₄
13. The molecular formula of a compound is a multiple of its ____.
    a) atomic formula
    b) empirical formula
    c) valency
    d) isotope formula
14. To balance the equation, 2H₂ + O₂ → ____, the product is ____.
    a) 2H₂O₂
    b) H₂O
    c) H₂O₂
    d) 2H₂O
15. The relative molecular mass of water (H₂O) is ____.
    a) 10
    b) 18
    c) 16
    d) 12

CLASS ACTIVITY DISCUSSION

1. What is the difference between an empirical formula and a molecular formula?
   • The empirical formula gives the simplest ratio of atoms, while the molecular formula gives the actual number of atoms in a molecule.
2. Why must chemical equations be balanced?
   • To comply with the law of conservation of mass, ensuring the same number of atoms on both sides of the equation.
3. What is a chemical formula?
   • It’s a combination of symbols that represent the elements in a compound.
4. What does the formula NaCl represent?
   • It represents one molecule of sodium chloride, containing one sodium atom and one chlorine atom.
5. What does it mean if a molecule is diatomic?
   • It means the molecule consists of two atoms, such as O₂ or H₂.
6. How can you find the molecular formula from the empirical formula?
   • By knowing the compound’s molecular mass and comparing it to the empirical formula mass.
7. What is the molecular formula of water?
   • The molecular formula of water is H₂O.
8. How do you calculate the relative molecular mass?
   • By adding up the atomic masses of each element in the formula.
9. What are examples of diatomic molecules?
   • Examples include O₂, H₂, and N₂.
10. How is a chemical reaction represented?
    • It is represented by a chemical equation showing reactants and products.
11. What does H₂SO₄ stand for?
    • It stands for sulfuric acid.
12. What is the difference between HCl and H₂S?
    • HCl is hydrogen chloride, and H₂S is hydrogen sulfide, each with different chemical properties.
13. Why is the empirical formula useful?
    • It helps to understand the basic composition of a compound without exact numbers of atoms.
14. What is a molar mass?
    • Molar mass is the mass of one mole of a substance, measured in grams.
15. How is the percentage composition by mass useful?
    • It helps determine the empirical formula and shows the distribution of elements in a compound.

EVALUATION

1. Differentiate between empirical and molecular formulae.
2. Balance the following equation: C₃H₈ + O₂ → CO₂ + H₂O.
3. What is the empirical formula of a compound that contains 70% Fe and 30% O?
4. State the formula of the following compounds: Calcium carbonate, Magnesium oxide, and Sulfur dioxide.
5. Calculate the molar mass of CO₂.
6. Why is a diatomic molecule like O₂ represented with a subscript 2?
7. Calculate the percentage composition of carbon in C₂H₅OH.
8. How many atoms are in one molecule of NH₃?
9. What is the symbol for the sodium ion in NaCl?
10. Calculate the relative molecular mass of H₂SO₄.

EVALUATION QUESTIONS

1. The chemical formula of oxygen gas is ____.
   a) O
   b) O₂
   c) O₃
   d) H₂O
2. The empirical formula shows ____.
   a) the exact number of atoms in a molecule
   b) the ratio of atoms in a compound
   c) the chemical name
   d) the molecular weight
3. A balanced chemical equation obeys the ____.
   a) law of gravity
   b) law of conservation of mass
   c) law of conservation of energy
   d) atomic theory
4. The molecular formula of methane is ____.
   a) CH₄
   b) CH₃
   c) C₂H₆
   d) CH₄O
5. The formula NaCl represents ____.
   a) sodium chloride
   b) sodium bicarbonate
   c) sodium carbonate
   d) sodium oxide
6. In the reaction 2H₂ + O₂ → 2H₂O, water is the ____.
   a) reactant
   b) product
   c) by-product
   d) catalyst
7. The simplest ratio of atoms in a compound is given by the ____.
   a) molecular formula
   b) empirical formula
   c) chemical name
   d) balanced equation
8. The empirical formula of hydrogen peroxide (H₂O₂) is ____.
   a) HO
   b) H₂O₂
   c) H₂O
   d) OH₂
9. The chemical formula for calcium chloride is ____.
   a) CaCl₂
   b) CaCl
   c) Ca₂Cl
   d) Cl₂Ca
10. To balance a chemical equation, you need to make sure that the number of ____ is the same on both sides.
    a) molecules
    b) atoms
    c) ions
    d) compounds
11. Which of the following is a molecular formula?
    a) CH
    b) CH₄
    c) C₃H
    d) H₂
12. The mass of one mole of carbon-12 is ____.
    a) 6 grams
    b) 12 grams
    c) 18 grams
    d) 24 grams
13. In NaCl, the symbol Na represents ____.
    a) neon
    b) nitrogen
    c) sodium
    d) sulfur
14. The empirical formula of glucose (C₆H₁₂O₆) is ____.
    a) C₂H₄O
    b) CH₂O
    c) C₃H₆O₃
    d) C₆H₁₂O₆
15. The chemical reaction between hydrogen and chlorine forms ____.
    a) H₂Cl
    b) H₂O
    c) HCl
    d) HCl₂

CLASS ACTIVITY DISCUSSION

1. What is the difference between an empirical formula and a molecular formula?
   • The empirical formula shows the simplest ratio of atoms, while the molecular formula shows the exact number of atoms in a molecule.
2. Why do we need to balance chemical equations?
   • To follow the law of conservation of mass, which states that mass cannot be created or destroyed in a chemical reaction.
3. What does the formula H₂O represent?
   • It represents water, with two hydrogen atoms and one oxygen atom.
4. What is a diatomic molecule?
   • A molecule consisting of two atoms, like O₂ or H₂.
5. Why is an empirical formula important?
   • It helps show the simplest proportions of elements in a compound.
6. What does a chemical formula represent?
   • It shows the elements and the number of atoms of each element in a compound.
7. How is the relative molecular mass calculated?
   • By adding the atomic masses of all atoms in a molecule.
8. What is an example of a molecular formula?
   • C₂H₆ for ethane.
9. What does NaCl represent?
   • Sodium chloride, commonly known as table salt.
10. Why are physical states important in chemical equations?
    • They indicate whether a substance is a solid, liquid, gas, or aqueous solution.
11. What does the formula CO₂ represent?
    • Carbon dioxide, with one carbon and two oxygen atoms.
12. How do you calculate molar mass?
    • By finding the relative molecular mass and expressing it in grams per mole.
13. Why is the molecular formula of a compound useful?
    • It provides the exact number of atoms of each element in a molecule.
14. How can you identify an empirical formula?
    • It’s the simplest ratio of elements in a compound, often different from the molecular formula.
15. What is the percentage composition by mass?
    • It’s the percentage of each element’s mass in a compound, useful for determining its empirical formula.

EVALUATION

1. What is the chemical formula for hydrogen chloride?
2. Write the empirical formula of a compound with 80% carbon and 20% hydrogen.
3. Balance the following equation: H₂ + O₂ → H₂O.
4. Define the term “relative molecular mass.”
5. What is the law of conservation of mass?
6. Find the molecular formula of a compound with an empirical formula of CH₂O and a molecular mass of 60 g/mol.
7. Differentiate between a molecule and a compound.
8. What is the empirical formula of C₆H₁₂O₆?
9. Calculate the molar mass of H₂SO₄.
10. Write the formula of magnesium oxide.

Revision and Review Test Questions Chemistry SS 1 First Term Lesson Notes Week 11