Atomic Foundations of Matter — Class 9 Science NCERT Solutions (Free)

Free step-by-step NCERT solutions for Class 9 Science chapter "Atomic Foundations of Matter" — 8 important questions with detailed answers for CBSE board exam preparation.

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TL;DR: Free step-by-step NCERT solutions for Class 9 Science chapter "Atomic Foundations of Matter" — 8 important questions with detailed answers for CBSE bo…

Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated Jul 23, 2026

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Key Questions Covered:

State and explain the Law of Conservation of Mass.
State and explain the Law of Definite Proportions.
Define atomic mass unit (u) and atomic mass.
What is the mole concept? Define mole and molar mass.
Calculate the number of moles in 36 g of water (H₂O).
What is a chemical formula? Explain empirical and molecular formula with exam…
+ 2 more questions in the full chapter

Solutions Summary:

Question	Status
State and explain the Law of Conservation of Mass.	✓ Solved
State and explain the Law of Definite Proportions.	✓ Solved
Define atomic mass unit (u) and atomic mass.	✓ Solved
What is the mole concept? Define mole and molar mass.	✓ Solved
Calculate the number of moles in 36 g of water (H₂O).	✓ Solved
What is a chemical formula? Explain empirical and molecul…	✓ Solved

Showing 6 of 8 questions

Q1: State and explain the Law of Conservation of Mass.

Law of Conservation of Mass: Mass cannot be created or destroyed in a chemical reaction; mass of reactants = mass of products. Explanation: In chemical reaction, bonds break and reform between atoms. Atoms rearrange but total number of each type of atom remains same, so total mass remains constant. Example: Hydrogen + Oxygen → Water 2H₂ + O₂ → 2H₂O 4 g + 32 g → 36 g (mass conserved)

Q2: State and explain the Law of Definite Proportions.

Law of Definite Proportions (Law of Constant Composition): A compound always contains the same elements in the same proportion by mass, regardless of source or method of preparation. Example: Water always contains H and O in ratio 1:8 by mass 2 g H : 16 g O in any water sample (H₂O) Explanation: A compound has fixed chemical formula determined by specific number of atoms of each element; ratio of atoms = ratio of masses.

Q3: Define atomic mass unit (u) and atomic mass.

Atomic mass unit (u or amu): Standard unit for measuring mass of atoms and molecules. 1 u = 1/12 of mass of one carbon-12 atom = 1.66 × 10⁻²⁷ kg Atomic mass = Average relative mass of atom of an element compared to 1/12 of C-12 Example: Atomic mass of Hydrogen ≈ 1 u, Carbon = 12 u, Oxygen = 16 u Molecular mass = Sum of atomic masses of all atoms in molecule Example: H₂O = 1 + 1 + 16 = 18 u

Q4: What is the mole concept? Define mole and molar mass.

Mole = Unit of amount of substance; 1 mole = 6.022 × 10²³ particles (Avogadro's number Nₐ) Molar mass (M) = Mass of one mole of substance in grams = Numerically equal to molecular/atomic mass Example: Molar mass of H₂O = 18 g/mol, O₂ = 32 g/mol Relationship: n = m/M = N/Nₐ where n = number of moles, m = mass (g), N = number of particles Applications: Convert between mass, moles, and number of particles

Q5: Calculate the number of moles in 36 g of water (H₂O).

Given: mass of water = 36 g Molar mass of H₂O = 2(1) + 16 = 18 g/mol Using formula: n = m/M n = 36/18 = 2 moles Alternatively: Number of molecules = n × Nₐ = 2 × 6.022 × 10²³ = 1.204 × 10²⁴ molecules

Q6: What is a chemical formula? Explain empirical and molecular formula with example.

Chemical formula = Symbolic representation of compound showing types and relative numbers of atoms of each element. Empirical formula = Simplest whole number ratio of atoms of different elements Molecular formula = Actual number of atoms of each element in one molecule Example: Glucose C₆H₁₂O₆ Empirical formula = CH₂O (ratio 1:2:1) Molecular formula = C₆H₁₂O₆ (actual number of atoms) Relationship: Molecular formula = (Empirical formula)ₙ

Showing 6 of 8 questions. Visit the full page for complete solutions.