What is the mole concept and molar mass in Class 9 Science?

A mole is a counting unit equal to 6.022x10^23 particles (Avogadro's number). The molar mass of a substance is the mass of one mole expressed in grams — numerically equal to its atomic or molecular mass.

What is mole concept & molar mass in Class 9 Science (CBSE/NCERT)?

Mole Concept & Molar Mass is a key topic in NCERT Class 9 Science Chapter 9 u2014 Atomic Foundations of Matter. It explains the mole concept, Avogadro's number, molar mass and converting between mass, moles and number of particles.

Why is mole important in NCERT Class 9 Science?

Mole is important because it forms the foundation for understanding mole concept & molar mass in Chapter 9. Without a clear idea of mole, students cannot answer higher-order CBSE questions involving Avogadro's number and molar mass.

How is mole concept & molar mass tested in the Class 9 Science CBSE exam?

CBSE tests this through 1-mark MCQs, 2-mark short answers, 3-mark explanations, 5-mark descriptive questions and 4-mark competency-based questions on mole, Avogadro's number and molar mass drawn from NCERT activities.

What are the key terms to remember for mole concept & molar mass in Class 9 Science?

Key terms: mole, Avogadro's number, 6.022 x 10^23, molar mass, atomic mass, molecular mass. Each carries exam weightage and regularly appears in the CBSE Class 9 paper.

Is Mole Concept & Molar Mass included in the Class 9 Science syllabus for 2025u201326 CBSE?

Yes, Mole Concept & Molar Mass is part of the NCERT Class 9 Science syllabus (2025u201326) under the new NCERT Exploration textbook. It falls under Chapter 9 and is examined in the annual paper.

How should I prepare mole concept & molar mass for the CBSE Class 9 Science exam?

Prepare in three steps. First, read this NCERT part carefully, highlighting definitions and diagrams of mole, Avogadro's number and molar mass. Second, solve every in-text question and end-of-chapter exercise. Third, practise competency-based and assertion-reason questions.

What is mole concept & molar mass in Class 9 Science (CBSE/NCERT)?

Mole Concept & Molar Mass is a key topic in NCERT Class 9 Science Chapter 9 — Atomic Foundations of Matter. It explains the mole concept, avogadro's number, molar mass and converting between mass, moles and number of particles. Core ideas covered include mole, Avogadro's number, 6.022 x 10^23, molar mass. Mastering this subtopic is essential for scoring well in the CBSE Class 9 Science exam and for building a strong foundation for the Class 10 board exam, because these concepts repeatedly appear in MCQs, short answers and long-answer questions. This part gives a complete, exam-ready explanation with activities, diagrams and competency-based practice aligned to NCERT.

Why is mole important in NCERT Class 9 Science?

Mole is important in NCERT Class 9 Science because it forms the foundation for understanding mole concept & molar mass in Chapter 9 — Atomic Foundations of Matter. Without a clear idea of mole, students cannot answer higher-order CBSE questions involving Avogadro's number, 6.022 x 10^23, molar mass. School and competitive papers regularly include 2-mark and 3-mark questions on this concept, and competency-based questions often link mole to real-life situations. Building clarity here pays off directly in marks at Class 9 and again in the Class 10 board exam.

How is mole concept & molar mass tested in the Class 9 Science CBSE exam?

The CBSE Class 9 Science exam tests mole concept & molar mass through a mix of 1-mark MCQs, 2-mark short answers, 3-mark explanations with examples, 5-mark descriptive questions (often with diagrams or derivations) and 4-mark competency-based questions. Expect direct questions on mole, Avogadro's number, 6.022 x 10^23 and application-based questions drawn from NCERT activities. Students who follow the NCERT Exploration textbook thoroughly and practise this chapter's questions consistently score in the 90%+ range.

What are the key terms to remember for mole concept & molar mass in Class 9 Science?

The key terms to remember for mole concept & molar mass in NCERT Class 9 Science Chapter 9 are: mole, Avogadro's number, 6.022 x 10^23, molar mass, atomic mass, molecular mass. Each of these concepts carries exam weightage and regularly appears in the CBSE Class 9 paper. Write clear one-line definitions of every term in your revision notes and revisit them before the exam. Linking these terms visually through a flowchart or concept map makes recall easier during the Class 9 Science exam.

Is Mole Concept & Molar Mass included in the Class 9 Science syllabus for 2025–26 CBSE?

Yes, Mole Concept & Molar Mass is part of the NCERT Class 9 Science syllabus (2025–26) prescribed by CBSE under the new NCERT Exploration textbook. It falls under Chapter 9 — Atomic Foundations of Matter — and is examined in the annual paper. The current syllabus retains the full treatment of mole, Avogadro's number, 6.022 x 10^23 as per the NCERT textbook. Because CBSE bases every Class 9 question on NCERT, studying this part thoroughly ensures complete syllabus coverage and guarantees marks from this chapter.

How should I prepare mole concept & molar mass for the CBSE Class 9 Science exam?

Prepare mole concept & molar mass for the CBSE Class 9 Science exam in three steps. First, read this NCERT part carefully, highlighting definitions and diagrams of mole, Avogadro's number, 6.022 x 10^23. Second, solve every in-text question and end-of-chapter exercise — CBSE questions often come directly from NCERT. Third, practise competency-based and assertion-reason questions to sharpen reasoning. Write answers in the exam-style format (point-wise with diagrams) and time yourself. This method delivers confidence and full marks in the exam.

TOPIC 33 OF 50

Mole Concept and Molar Mass

🎓 Class 9 Science CBSE Theory Ch 9 — Atomic Foundations of Matter ⏱ ~14 min

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Introduction: Counting the Uncountable

If a goldsmith hands you a single grain of gold dust, you cannot point to a single atom and say "there it is" — even that grain holds well over a billion billion atoms. Yet chemistry is all about counting atoms, because reactions happen one atom (or one molecule) at a time. How do chemists handle such fantastically large numbers? They use a counting unit called the mole, in much the same way we use "a dozen" for 12 things or "a ream" for 500 sheets of paper.

Big Idea: The mole is a chemist's "dozen" — but for atoms and molecules. It bridges the world of single tiny particles with the macroscopic world of grams that we can actually weigh.

9.8 The Mole Concept

By international agreement, one mole of any substance contains the same definite number of particles. That number is famously called Avogadro's number, in honour of the Italian scientist Amedeo Avogadro.

Avogadro's Number. \( N_A = 6.022 \times 10^{23} \) particles per mole. So one mole of any element or compound contains \(6.022 \times 10^{23}\) atoms, molecules or formula units of that substance.

1 mole of carbon = \(6.022 \times 10^{23}\) carbon atoms
1 mole of water = \(6.022 \times 10^{23}\) water molecules
1 mole of Na⁺ ions = \(6.022 \times 10^{23}\) sodium ions

Fig 9.8: One mole of any substance always has Avogadro's number of particles, but the mass changes from substance to substance.

9.9 Molar Mass

The mole is cleverly defined so that the mass of one mole of a substance, measured in grams, is numerically equal to its atomic or molecular mass measured in u.

Molar mass. The mass (in grams) of one mole of a substance is called its molar mass. It is numerically equal to the atomic mass (for an element) or the molecular mass (for a compound), with the unit changed from u to g/mol.

Substance	Atomic / Molecular mass	Molar mass
Hydrogen atom (H)	1 u	1 g/mol
Oxygen atom (O)	16 u	16 g/mol
Sodium atom (Na)	23 u	23 g/mol
Hydrogen gas (H₂)	2 u	2 g/mol
Water (H₂O)	18 u	18 g/mol
Carbon dioxide (CO₂)	44 u	44 g/mol
Sodium chloride (NaCl)	58.5 u	58.5 g/mol

Molecular mass — how to compute

The molecular mass of a compound is the sum of the atomic masses of all the atoms in its molecule.

H₂O: 2(1) + 16 = 18 u → 18 g/mol
CO₂: 12 + 2(16) = 44 u → 44 g/mol
H₂SO₄: 2(1) + 32 + 4(16) = 98 u → 98 g/mol
Ca(OH)₂: 40 + 2(16+1) = 40 + 34 = 74 u → 74 g/mol

9.10 The Mass–Mole–Particle Triangle

Three quantities of a sample are linked by two simple equations: the mass m, the number of moles n, and the number of particles N. If M is the molar mass and N_A is Avogadro's number,

\[ n = \dfrac{m}{M} \qquad N = n \times N_A \qquad m = n \times M \]

🔺 Mass ↔ Moles ↔ Particles — Step through a worked conversion L3 Apply

Click each corner of the triangle in order — start with Mass, then Moles, then Particles — and apply the formula at each step using a worked example: 11 g of CO₂.

Fig 9.9: The mole sits at the centre. Convert mass to moles by dividing by molar mass; convert moles to number of particles by multiplying by Avogadro's number.

Click Mass → Moles → Particles in order to walk through a worked conversion (example: 11 g of CO₂).

Activity — Visualising one mole

Activity 9.3 — How Big Is a Mole?L4 Analyse

Predict first: If a single grain of rice weighs about 25 mg, how massive would 1 mole (6.022×10²³) of grains of rice be? Take a guess before you compute.

Mass of 1 grain = 25 mg = \(25 \times 10^{-3}\) g.
Mass of \(6.022 \times 10^{23}\) grains = \(25 \times 10^{-3} \times 6.022 \times 10^{23}\) g.
Convert to kilograms and tonnes.

Mass = \(1.51 \times 10^{22}\) g = \(1.51 \times 10^{19}\) kg = \(1.51 \times 10^{16}\) tonnes — far greater than the total food produced on Earth in a million years!

Take-home: Avogadro's number is staggeringly large. That is why one mole of atoms weighs only a few grams — atoms themselves are unimaginably tiny.

9.11 Worked Numericals

The atomic masses you will need: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, S = 32, Cl = 35.5, K = 39, Ca = 40, Fe = 56, Cu = 63.5. Use \(N_A = 6.022 \times 10^{23}\).

Numerical 1 Find the number of moles in 36 g of water.

Given: mass \(m = 36\) g; \(M(\text{H}_2\text{O}) = 2(1) + 16 = 18\) g/mol.

Formula: \( n = \dfrac{m}{M} \).

Calculation: \( n = \dfrac{36}{18} = 2 \) mol. Answer: 2 moles.

Numerical 2 What is the mass of 0.25 mole of carbon dioxide?

Given: \(n = 0.25\) mol; \(M(\text{CO}_2) = 12 + 2(16) = 44\) g/mol.

Formula: \( m = n \times M \).

Calculation: \( m = 0.25 \times 44 = 11 \) g. Answer: 11 g.

Numerical 3 Calculate the number of molecules in 11 g of CO₂.

Step 1 — moles: \( n = \dfrac{11}{44} = 0.25 \) mol.

Step 2 — molecules: \( N = n \times N_A = 0.25 \times 6.022 \times 10^{23} \).

Answer: \( N = 1.5055 \times 10^{23} \approx 1.51 \times 10^{23} \) molecules.

Numerical 4 How many atoms are present in 4 g of helium gas? (atomic mass of He = 4)

Step 1: Helium is mono-atomic, \(M = 4\) g/mol. Moles = \( 4/4 = 1 \) mol.

Step 2: Atoms = \(1 \times 6.022 \times 10^{23} = 6.022 \times 10^{23}\) atoms.

Numerical 5 Find the mass of 3.011 × 10²³ atoms of sodium.

Step 1 — moles: \( n = \dfrac{3.011 \times 10^{23}}{6.022 \times 10^{23}} = 0.5 \) mol.

Step 2 — mass: \( m = n \times M = 0.5 \times 23 = 11.5 \) g.

Numerical 6 A sample contains 0.2 mole of nitric acid. Find (a) its mass and (b) the number of HNO₃ molecules.

Molar mass: HNO₃ = 1 + 14 + 3(16) = 63 g/mol.

(a) \( m = 0.2 \times 63 = 12.6 \) g.

(b) \( N = 0.2 \times 6.022 \times 10^{23} = 1.2044 \times 10^{23} \) molecules.

Numerical 7 Calculate the number of moles in 24 g of magnesium and the number of atoms it contains.

Moles: \( n = 24/24 = 1 \) mol.

Atoms: \( N = 1 \times 6.022 \times 10^{23} = 6.022 \times 10^{23} \) atoms.

Numerical 8 What mass of sodium hydroxide (NaOH) contains the same number of formula units as 9 g of water?

Step 1: Moles in 9 g H₂O = \( 9/18 = 0.5 \) mol → contains \(0.5 \times N_A\) molecules.

Step 2: Same particle count means 0.5 mole of NaOH. Molar mass NaOH = 23 + 16 + 1 = 40 g/mol.

Step 3: Mass NaOH = \( 0.5 \times 40 = 20 \) g.

Numerical 9 Find the number of oxygen atoms (not molecules) in 16 g of dioxygen gas.

Step 1 — moles of O₂: \( n = 16/32 = 0.5 \) mol.

Step 2 — O₂ molecules: \( 0.5 \times 6.022 \times 10^{23} = 3.011 \times 10^{23} \) molecules.

Step 3 — atoms: Each O₂ molecule has 2 atoms → \( 2 \times 3.011 \times 10^{23} = 6.022 \times 10^{23} \) O atoms.

Numerical 10 Calculate the mass of one molecule of water in grams.

Logic: 1 mole = \(6.022 \times 10^{23}\) molecules has a mass of 18 g.

Mass of one molecule: \( m_1 = \dfrac{18}{6.022 \times 10^{23}} \).

Answer: \( m_1 \approx 2.99 \times 10^{-23} \) g per molecule.

Quick Recap

Quantity	Symbol / Formula	Unit
Mole	\( n \)	mol
Avogadro's number	\( N_A = 6.022 \times 10^{23} \)	per mol
Molar mass	\( M \) (= atomic/molecular mass in g/mol)	g/mol
Number of particles	\( N = n \times N_A \)	—
Moles from mass	\( n = m/M \)	mol

Competency-Based Questions

A pharmacy stocks a bottle that contains 9.8 g of pure sulphuric acid (H₂SO₄). Atomic masses: H = 1, S = 32, O = 16. Use \(N_A = 6.022 \times 10^{23}\).

Q1. The molar mass of H₂SO₄ is: L1

(a) 49 g/mol
(b) 96 g/mol
(c) 98 g/mol
(d) 100 g/mol

(c) 98 g/mol. 2(1) + 32 + 4(16) = 2 + 32 + 64 = 98.

Q2. How many moles of H₂SO₄ are present in the bottle? L3

\( n = m/M = 9.8/98 = 0.1 \) mol.

Q3. Find the number of H₂SO₄ molecules in the bottle. L3

\( N = n \times N_A = 0.1 \times 6.022 \times 10^{23} = 6.022 \times 10^{22}\) molecules.

Q4. State whether True or False with justification: "1 mole of H₂ and 1 mole of O₂ contain the same number of molecules but different masses." L2

True. Each contains \(N_A\) molecules. But masses are different — H₂ is 2 g while O₂ is 32 g, because O atoms are heavier than H atoms.

Q5. Fill in the blank: The mass of \(3.011 \times 10^{23}\) atoms of carbon is ____ g. L3

\(3.011 \times 10^{23}\) atoms = 0.5 mol → mass \(= 0.5 \times 12 = 6\) g.

Assertion–Reason Questions

Options: (A) Both A and R are true and R is the correct explanation of A. (B) Both true but R is not the correct explanation. (C) A true, R false. (D) A false, R true.

A: One mole of any gas has the same number of molecules at the same temperature and pressure.

R: One mole of every substance contains \(6.022 \times 10^{23}\) elementary entities by definition.

(A) Both true and R is the correct explanation. The mole is defined precisely so that this universal counting works.

A: 22 g of carbon dioxide and 18 g of water contain the same number of molecules.

R: Both samples represent 0.5 mole and 1 mole respectively.

(D) Assertion is false — 22 g CO₂ = 0.5 mol while 18 g water = 1 mol, so they do NOT contain equal molecules. Reason is true and proves that the assertion is wrong.

A: The molar mass of a substance is numerically equal to its molecular mass.

R: The mole is defined so that the mass in grams of \(N_A\) particles equals the mass in u of one particle.

(A) Both true and R correctly explains A — that is precisely the design of the mole concept.

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