2nd Term, week 2 Chemistry Lesson Note: Mole Concept and Stoichiometry (SSS 1)

Term: 2nd Term

Week: 2

Class: Senior Secondary School 1 (SSS 1)

Subject: Chemistry

Topic: Mole Concept and Stoichiometry

Sub-Topic: Introduction to the Mole, Molar Mass, and Avogadro’s Number

I. Learning Objectives

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

Explain the mole as the chemist's counting unit for particles.
State the value of Avogadro’s number ( $N_A$ ).
Differentiate between atomic mass and molar mass.
Calculate the molar mass of simple elements and compounds.
Convert given masses of substances into moles (and vice versa).

II. Lesson Content

1. The Concept of Counting in Chemistry (The Mole)

In everyday life, we group items together into counting units to make them easier to handle:

1 Dozen = 12 items (e.g., a dozen eggs, a dozen books)
1 Pair = 2 items (e.g., a pair of shoes)
1 Ream = 500 sheets of paper

In chemistry, the particles we deal with (atoms, molecules, and ions) are too tiny to be counted individually. Therefore, chemists use a large collective counting unit called the Mole (abbreviated as mol).

The Mole: The mole is the SI unit for measuring the amount of a chemical substance. Just as "1 dozen" always means 12, "1 mole" always means a specific, huge number of chemical particles.

2. Avogadro’s Number ( $N_A$ )

The specific number of particles in exactly one mole of any substance is a constant named after the scientist Amedeo Avogadro.

\text{Avogadro's Number } (N_A) = 6.02 \times 10^{23} \text{ particles per mole}

No matter what the substance is, one mole of it will always contain $6.02 \times 10^{23}$ particles.

$1\text{ mole of Sodium (Na) atoms} = 6.02 \times 10^{23} \text{ atoms}$
$1\text{ mole of Water } (\text{H}_2\text{O}) \text{ molecules} = 6.02 \times 10^{23} \text{ molecules}$
$1\text{ mole of Chlorine } (\text{Cl}^-) \text{ ions} = 6.02 \times 10^{23} \text{ ions}$

3. Molar Mass ( $M$ )

Think about this: Does a dozen chicken eggs weigh the same as a dozen plastic balls? No. Even though the count is the same (12), their weights are different because the individual items have different masses.

Similarly, one mole of Carbon atoms contains the same number of particles as one mole of Lead atoms, but they have completely different masses.

Molar Mass: This is the mass of one mole of a substance. Its unit of measurement is grams per mole ( $\text{g/mol}$ ).

For Elements: The molar mass is simply the Relative Atomic Mass ( $A_r$ ) of the element taken from the Periodic Table, expressed in grams.
For Compounds: The molar mass is calculated by adding up the relative atomic masses of all the individual atoms present in the chemical formula.

The Magic Formula:

To find the relationship between mass, molar mass, and moles, we use:

\text{Number of Moles } (n) = \frac{\text{Mass of substance in grams } (m)}{\text{Molar Mass of substance } (M)}

III. Worked Examples

Example 1: Calculating Molar Mass of a Compound

Calculate the molar mass of Sodium Hydroxide ( $\text{NaOH}$ ).

(Given Relative Atomic Masses: $\text{Na} = 23, \text{O} = 16, \text{H} = 1$

\text{Molar Mass of } \text{NaOH} = (\text{Mass of Na}) + (\text{Mass of O}) + (\text{Mass of H})

\text{Molar Mass} = 23 + 16 + 1 = 40\text{ g/mol}

Example 2: Converting Mass to Moles

How many moles are present in $10\text{ g}$ of Calcium ( $\text{Ca}$ )?

(Given Atomic Mass: $\text{Ca} = 40$ )

Example 3: Converting Moles to Mass

What is the mass in grams of $2.5\text{ moles}$ of Water ( $\text{H}_2\text{O}$

(Given Atomic Masses: $\text{H} = 1, \text{O} = 16$ )

First, find the molar mass of $\text{H}_2\text{O}$ :
$\text{Molar Mass} = (1 \times 2) + 16 = 2 + 16 = 18\text{ g/mol}$
Next, rearrange the formula to find Mass:
$\text{Mass } (m) = \text{Number of Moles } (n) \times \text{Molar Mass } (M)$
$\text{Mass} = 2.5\text{ moles} \times 18\text{ g/mol} = 45\text{ grams}$

IV. Class Evaluation / Review Questions

Define the term mole in your own words.
What is the value of Avogadro's constant?
Calculate the molar mass of the following:
- (a) Magnesium oxide ( $\text{MgO}$ )
- (b) Carbon dioxide ( $\text{CO}_2$ )
  (Atomic masses: $\text{Mg}=24, \text{O}=16, \text{C}=12$ )
Calculate the number of moles in $14\text{ g}$ of Nitrogen atoms ( $\text{N}=14$ ).

V. Homework Assignment

Calculate the molar mass of Propane gas ( $\text{C}_3\text{H}_8$ ). ( $\text{C}=12, \text{H}=1$ )
How many moles are contained in $100\text{ g}$ of Calcium Carbonate ( $\text{CaCO}_3$ ) ( $\text{Ca}=40, \text{C}=12, \text{O}=16$ )
Find the mass of $0.5\text{ moles}$ of Sodium Chloride ( $\text{NaCl}$ ). ( $\text{Na}=23, \text{Cl}=35.5$ )

Exam solutions

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