This MCQ module is based on: Atomic Foundations of Matter — NCERT Exercises
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Atomic Foundations of Matter — NCERT Exercises
🎓 Class 9
Science
CBSE
Theory
Ch 9 — Atomic Foundations of Matter
⏱ ~15 min
🌐 Language: [gtranslate]
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Chapter Summary
Chapter 9 builds the atomic and molecular foundations of chemistry. Two experimental laws (conservation of mass, constant proportions) led Dalton to propose that matter is made of indivisible atoms. From there, chemists defined atomic mass, molecules, ions, chemical formulas, and finally the mole — the bridge between counting particles and weighing samples.
Conservation of MassTotal mass of reactants equals total mass of products. Lavoisier (1789).
Constant ProportionsA pure compound has a fixed mass ratio of its elements regardless of source. Proust (1799).
Dalton's Atomic TheoryAtoms are indivisible; identical within an element; combine in whole-number ratios; never created or destroyed in a reaction.
Atomic Mass1 u = 1/12 mass of a C-12 atom. H = 1 u, O = 16 u, etc.
MoleculeSmallest particle of a substance that can exist independently. Atomicity = atoms per molecule.
IonsCations (Na⁺, Mg²⁺, Al³⁺) — atoms that lost electrons; anions (Cl⁻, O²⁻, OH⁻, SO4²⁻) — atoms/groups that gained electrons.
Chemical FormulaUse the criss-cross method on valencies/charges to write Al2(SO4)3, MgCl2, etc.
Mole1 mole = \(6.022 \times 10^{23}\) particles (Avogadro's number).
Molar MassMass of 1 mole in grams; numerically equal to atomic/molecular mass in u.
Conversion Triangle\(n = m/M\); \(N = n \cdot N_A\); \(m = n \cdot M\).
Keyword Grid
AtomSmallest indivisible chemical unit of an element.
MoleculeGroup of atoms able to exist independently.
AtomicityNumber of atoms in one molecule.
CationPositive ion (loses e⁻).
AnionNegative ion (gains e⁻).
ValencyCombining capacity of an element.
Atomic Mass Unit1 u = 1/12 of C-12 mass.
Avogadro's Number\(6.022 \times 10^{23}\) per mole.
MoleSI unit of amount of substance.
Molar MassMass of 1 mole in g/mol.
Conservation of MassNo mass change during reaction.
Constant ProportionsFixed mass ratio in a compound.
Polyatomic IonCharged group of atoms (SO4²⁻, NH4⁺).
Chemical FormulaSymbolic representation of one formula unit.
Criss-Cross MethodSwap valencies to subscripts.
NCERT Exercises — Step-by-Step Solutions
Click "Show Solution" to view the worked answer. Try each problem yourself before peeking. Use atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, S = 32, Cl = 35.5, K = 39, Ca = 40. \(N_A = 6.022 \times 10^{23}\).
Q 1 State the law of conservation of mass and explain how it is verified by an experiment.
Statement: In any chemical reaction, the total mass of the reactants is equal to the total mass of the products. Mass is neither created nor destroyed during a chemical change.
Verification: Take Na2SO4 solution in an ignition tube and BaCl2 solution in a conical flask. Cork the flask tightly and weigh the apparatus. Tilt the flask so the two solutions mix; a white BaSO4 precipitate forms. Reweigh the (still corked) flask — the reading is unchanged, confirming the law.
Q 2 In a reaction, 5.3 g of sodium carbonate reacted with 6 g of acetic acid to give 8.2 g of sodium acetate, 2.2 g of carbon dioxide and 0.9 g of water. Show that this is consistent with the law of conservation of mass.
Mass of reactants = 5.3 + 6 = 11.3 g.
Mass of products = 8.2 + 2.2 + 0.9 = 11.3 g.
Reactant mass = product mass, so the law of conservation of mass is obeyed.
Q 3 Hydrogen and oxygen combine in the ratio 1 : 8 by mass to form water. What mass of oxygen would be required to react completely with 3 g of hydrogen?
By the law of constant proportions, mass of O = 8 × mass of H.
For 3 g of H: mass of O \(= 3 \times 8 = 24\) g.
Answer: 24 g of oxygen.
Q 4 Which postulate of Dalton's atomic theory explains the law of conservation of mass, and which one explains the law of constant proportions?
Conservation of mass is explained by the postulate that atoms are neither created nor destroyed in a chemical reaction — atoms only rearrange.
Constant proportions is explained by the postulate that atoms combine in small whole-number ratios. Since each atom has a fixed mass, the mass ratio of the elements is also fixed.
Q 5 Define the atomic mass unit. Why is C-12 chosen as the standard?
One atomic mass unit is defined as one-twelfth of the mass of an atom of carbon-12. \(1\,\text{u} = \dfrac{1}{12}m(^{12}\text{C}) \approx 1.66 \times 10^{-24}\) g.
C-12 was adopted as the reference because carbon is abundant, easily handled, forms a wide variety of compounds, and the choice gives most other elements convenient near-integer atomic masses. The decision was made by IUPAC and IUPAP in 1961 to unify the chemists' and physicists' mass scales.
Q 6 Define atomicity. State the atomicity of helium, ozone, phosphorus and sulphur.
Atomicity is the number of atoms present in one molecule of a substance.
Helium (He) — 1 (mono-atomic). Ozone (O3) — 3 (tri-atomic). Phosphorus (P4) — 4 (tetra-atomic). Sulphur (S8) — 8 (octa-atomic).
Q 7 Distinguish between a cation and an anion. Give two examples of each.
Cation — positively charged ion formed when an atom loses electrons. Examples: Na⁺, Mg²⁺.
Anion — negatively charged ion formed when an atom gains electrons. Examples: Cl⁻, O²⁻.
Cations are usually formed by metals, anions by non-metals.
Q 8 Write the chemical formulas of the following compounds using the criss-cross method: (i) magnesium chloride, (ii) calcium oxide, (iii) aluminium sulphate, (iv) ammonium nitrate, (v) sodium carbonate.
(i) Mg+2, Cl−1 → MgCl2.
(ii) Ca+2, O−2 → Ca2O2, simplify → CaO.
(iii) Al+3, SO4−2 → Al2(SO4)3.
(iv) NH4+1, NO3−1 → NH4NO3.
(v) Na+1, CO3−2 → Na2CO3.
Q 9 Calculate the molecular mass of: (a) H2O, (b) CO2, (c) HNO3, (d) Ca(OH)2, (e) NH3.
(a) H2O: 2(1) + 16 = 18 u.
(b) CO2: 12 + 2(16) = 44 u.
(c) HNO3: 1 + 14 + 3(16) = 63 u.
(d) Ca(OH)2: 40 + 2(16+1) = 40 + 34 = 74 u.
(e) NH3: 14 + 3(1) = 17 u.
Q 10 Define mole. State Avogadro's number. What is the molar mass of (i) oxygen molecule (O2) and (ii) sodium chloride (NaCl)?
Mole: The amount of a substance that contains as many elementary entities (atoms, molecules or ions) as there are atoms in exactly 12 g of carbon-12 — that is, \(6.022 \times 10^{23}\) particles.
Avogadro's number: \(N_A = 6.022 \times 10^{23}\) per mole.
(i) Molar mass of O2 = 2(16) = 32 g/mol.
(ii) Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol.
Q 11 Calculate the number of moles in 5.85 g of sodium chloride.
Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol.
\( n = \dfrac{m}{M} = \dfrac{5.85}{58.5} = 0.1 \) mol.
Answer: 0.1 mole.
Q 12 Find the mass of 0.5 mole of nitrogen gas (N2) and the number of N2 molecules it contains.
Molar mass of N2 = 2(14) = 28 g/mol.
Mass = \(n \times M = 0.5 \times 28 = 14\) g.
Number of molecules = \(n \times N_A = 0.5 \times 6.022 \times 10^{23} = 3.011 \times 10^{23}\) molecules.
Q 13 Calculate the number of atoms present in (a) 12 g of carbon, (b) 4 g of helium, (c) 8 g of oxygen gas (O2) — count individual atoms, not molecules.
(a) 12 g C = 1 mol → \(6.022 \times 10^{23}\) atoms.
(b) 4 g He = 1 mol (helium is mono-atomic) → \(6.022 \times 10^{23}\) atoms.
(c) 8 g O2 = 8/32 = 0.25 mol of O2 → \(0.25 \times 6.022 \times 10^{23} = 1.5055 \times 10^{23}\) molecules. Each molecule has 2 atoms, so total O atoms = \(2 \times 1.5055 \times 10^{23} = 3.011 \times 10^{23}\).
Q 14 What mass of calcium carbonate (CaCO3) contains the same number of formula units as 18 g of water?
Moles in 18 g H2O = 18/18 = 1 mol → contains \(N_A\) molecules.
Same number of formula units ⇒ 1 mole of CaCO3.
Molar mass of CaCO3 = 40 + 12 + 3(16) = 100 g/mol.
Mass = \(1 \times 100 = 100\) g.
Q 15 Compute the mass of a single atom of nitrogen, in grams.
1 mole of N atoms has a mass of 14 g and contains \(6.022 \times 10^{23}\) atoms.
Mass of one atom = \( \dfrac{14}{6.022 \times 10^{23}} \).
\(= 2.33 \times 10^{-23}\) g per atom.
Frequently Asked Questions — NCERT Exercises & Intext Questions
How do I solve NCERT Class 9 Science Chapter 9 (Atomic Foundations of Matter) exercise questions for the CBSE board exam?
Solve NCERT Chapter 9 — Atomic Foundations of Matter — exercise questions by first reading the question carefully, writing down the given data, recalling the relevant concepts like laws of chemical combination, atom, molecule, and applying them step by step. This Part 4 covers every intext and end-of-chapter exercise from the NCERT textbook. Write balanced equations, label diagrams clearly and show each step — CBSE Class 9 examiners award step marks even if the final answer has a small slip. Practising these solutions strengthens conceptual clarity and builds speed for both the school exam and the upcoming Class 10 board exam.
Are the NCERT intext questions from Atomic Foundations of Matter important for the Class 9 Science exam?
Yes, NCERT intext questions for Chapter 9 Atomic Foundations of Matter are highly important for the CBSE Class 9 Science exam. Many questions in school and competitive papers are directly lifted or only slightly modified from these intext questions, and they test the foundational concepts — laws of chemical combination, atom, molecule — that chapter-end questions and the Class 10 board build on. Attempt every intext question first, then move on to the exercises. This practice ensures complete NCERT coverage, which is the CBSE syllabus's primary source.
What types of questions from Atomic Foundations of Matter are asked in the Class 9 Science exam?
The Class 9 Science paper (CBSE pattern) asks a mix of question types from Atomic Foundations of Matter: 1-mark MCQ and assertion-reason, 2-mark short answers, 3-mark explanations, 5-mark long answers with diagrams or derivations, and 4-mark competency-based / case-study questions. These test understanding of laws of chemical combination, atom, molecule, ion. Practising every NCERT exercise and intext question prepares you to answer all of these formats with confidence.
How many marks does Chapter 9 — Atomic Foundations of Matter — typically carry in the Class 9 Science paper?
Chapter 9 — Atomic Foundations of Matter — is part of the CBSE Class 9 Science syllabus and typically contributes 5–9 marks in the annual paper, depending on the year's weightage. Questions are drawn from definitions, reasoning, numerical/descriptive problems and diagrams on topics like laws of chemical combination, atom, molecule. Solving the NCERT exercises in this part is essential because CBSE directly references the NCERT Exploration textbook for question design.
Where can I find step-by-step NCERT solutions for Chapter 9 Atomic Foundations of Matter Class 9 Science?
You can find complete, step-by-step NCERT solutions for Chapter 9 Atomic Foundations of Matter Class 9 Science on MyAiSchool. Every intext and end-of-chapter exercise question is solved with full working, labelled diagrams and CBSE-aligned mark distribution. Solutions highlight key points about laws of chemical combination, atom, molecule that examiners look for. This makes revision quick and exam-focused for Class 9 CBSE students.
What is the best way to revise Atomic Foundations of Matter for the Class 9 Science exam?
The best way to revise Atomic Foundations of Matter for the CBSE Class 9 Science exam is a three-pass approach. First pass: skim the chapter and note down key terms like laws of chemical combination, atom, molecule in a one-page mind map. Second pass: solve every NCERT intext and exercise question without looking at the solution, then self-check. Third pass: attempt sample papers and competency-based questions under timed conditions. This structured revision secures full marks for this chapter.
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