Chapter 3: Atoms and Molecules Application Questions | science Class 9 CBSE

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Chapter 3: Atoms and Molecules Application Questions | science Class 9 CBSE | ByteNotes.in

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Question 1

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Q1. State any five postulates of Dalton’s atomic theory.

All matter is made up of indivisible particles called atoms.
Atoms of a given element are identical in mass and properties.
Atoms of different elements differ in mass and chemical properties.
Atoms combine in simple whole-number ratios to form compounds.
Atoms cannot be created or destroyed in chemical reactions.

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Question 2

Applied Concept

Q2. Define atomic mass and how it is expressed.

Atomic mass is the mass of a single atom of an element.
It is expressed in unified atomic mass units (u).
1 u is 1/12th the mass of a carbon-12 atom.
Used to calculate molecular or molar masses.
Relative, not absolute, mass is used.

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Question 3

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Q3. Write rules for writing a chemical formula.

Symbols of elements are used.
Cation (positive ion) is written first, then anion.
Valencies are criss-crossed to balance charges.
Polyatomic ions are bracketed if repeated.
Simplify subscripts to lowest whole numbers.

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Question 4

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Q4. Define molecular mass. How is it calculated?

It is the sum of atomic masses of all atoms in a molecule.
Use atomic mass values from periodic table.
Example: H₂O = 2(1) + 16 = 18 u.
Expressed in atomic mass units (u).
Helps in stoichiometric calculations.

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Question 5

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Q5. Define formula unit mass with example.

It is the sum of atomic masses in an ionic compound's formula.
Used for compounds with ionic bonds.
Example: NaCl = Na (23) + Cl (35.5) = 58.5 u.
Ionic compounds don't have discrete molecules.
Helps compare masses of ionic compounds.

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Question 6

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Q6. What is a mole? Define Avogadro’s constant.

Mole is the quantity of a substance containing 6.022 × 10²³ particles.
It is a basic SI unit for amount of substance.
Avogadro’s constant = 6.022 × 10²³.
Relates atomic/molecular mass to measurable quantities.
Widely used in chemical equations and lab work.

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Chapter 3: Atoms and Molecules | Application Questions

Q1. State any five postulates of Dalton’s atomic theory.

Q2. Define atomic mass and how it is expressed.

Q3. Write rules for writing a chemical formula.

Q4. Define molecular mass. How is it calculated?

Q5. Define formula unit mass with example.

Q6. What is a mole? Define Avogadro’s constant.

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