📖 Lesson 6: Mole Concept & Molar Calculations

👩🏽‍🏫 Introduction

Hey science problem-solver! 🧠

Imagine trying to count the number of grains of rice in a bag—you’d be there all day! Now imagine counting atoms, which are way smaller than rice. That’s impossible, right?

Well, chemists don’t count atoms one-by-one. Instead, they use a unit called the mole—a “chemist’s dozen”—to deal with incredibly large numbers of particles. Today, we’ll learn what a mole is, how to calculate molar masses, and how to solve simple mole problems.

📘 Core Concepts

🔹 What is a Mole?

A mole is a unit used to count atoms, molecules, or ions.

🧠 1 mole = 6.02 × 10²³ particles (Avogadro’s Number)

That’s 602,000,000,000,000,000,000,000 particles!

• 1 mole of water molecules = 6.02 × 10²³ molecules of H₂O

• 1 mole of sodium atoms = 6.02 × 10²³ Na atoms

🔹 Molar Mass

Molar mass is the mass of 1 mole of a substance. It’s equal to the relative atomic or molecular mass in grams.

🧠 Units: grams per mole (g/mol)

Examples:

🔹 Basic Formula: Moles, Mass, Molar Mass

🧠 Moles (n) = Mass (g) ÷ Molar Mass (g/mol)

Rearranged versions:

• Mass = Moles × Molar Mass

• Molar Mass = Mass ÷ Moles

👩🏽‍🔬 Real-Life Examples

• 1 mole of water = 18 grams = one tablespoon

• 1 mole of oxygen gas (O₂) = 32 g = air in a balloon

• Chemists use this for reactions, so they know how much of each substance to use or expect

🧩 Sample Problem Walkthrough

🧠 Problem 1:

Calculate the number of moles in 36 grams of water (H₂O).

Step-by-Step Solution:

1. Molar mass of H₂O = 2(1) + 16 = 18 g/mol

2. Moles = Mass ÷ Molar Mass = 36 ÷ 18 = 2 moles

🧠 Problem 2:

What is the mass of 0.5 moles of sodium chloride (NaCl)?

Step-by-Step Solution:

1. Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol

2. Mass = Moles × Molar Mass = 0.5 × 58.5 = 29.25 grams

✍🏽 Practice Exercises

Exercise 1: Fill-in-the-Blanks

1. One mole contains ______ particles.

2. The unit of molar mass is ______.

3. The formula for calculating moles is: Moles = ______ ÷ ______

Answers:

1. 6.02 × 10²³

2. grams per mole (g/mol)

3. Mass, Molar Mass

Exercise 2: Multiple Choice

1. What is the molar mass of carbon dioxide (CO₂)?
   A. 28 g/mol
   B. 44 g/mol
   C. 32 g/mol
   D. 36.5 g/mol

✅ Answer: B. 44 g/mol (12 + 2×16)

2. If you have 12 g of carbon (C), how many moles do you have?
   A. 0.5
   B. 1
   C. 2
   D. 6

✅ Answer: B. 1 (12 ÷ 12 = 1)

Exercise 3: Solve and Complete

Answers:

• H₂O: 90 ÷ 18 = 5 moles

• O₂: 64 ÷ 32 = 2 moles

• NaCl: 117 ÷ 58.5 = 2 moles

🌀 Recap

Let’s summarize:

• A mole represents 6.02 × 10²³ particles.

• Molar mass is the mass of one mole of a substance in grams.

• To calculate moles:

  Moles = Mass ÷ Molar Mass

• The mole helps chemists measure tiny particles in practical amounts for lab and industry.

💭 Reflection Prompt

Think of baking a cake. If your recipe needs 2 cups of flour and 1 cup of sugar, and you only have a tablespoon—how do you scale it? The mole is just like that—it helps us “scale” atoms and molecules.
Write one way you think the mole concept is useful in daily life or industry.