Atoms and Molecules — Previous Year Questions (Class 9 Science)
Atoms are the smallest units of elements, while molecules are combinations of atoms. Learn about atomic structure, compounds, and the mole concept.
TL;DR: Atoms are the smallest units of elements, while molecules are combinations of atoms. Learn about atomic structure, compounds, and the mole concept.
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Atoms are the smallest units of elements, while molecules are combinations of atoms. Learn about atomic structure, compounds, and the mole concept.
Atoms and Molecules — Previous Year Questions with Solutions
Q (2023, 3 marks): Calculate the number of atoms in 12 g of carbon-12.
Answer: Atomic mass of C-12 = 12 u
Molar mass of C-12 = 12 g/mol
Number of moles = Mass / Molar mass = 12 g / 12 g/mol = 1 mol
Avogadro's number = 6.022 × 10^23
Number of atoms = 1 × 6.022 × 10^23 = 6.022 × 10^23 atoms
Q (2022, 3 marks): Distinguish between elements, compounds, and mixtures with examples.
Answer: Element: Pure substance with only one type of atom. Example: Fe (iron), O2 (oxygen gas)
Compound: Pure substance formed by combining two or more elements in fixed ratio. Example: H2O (water), NaCl (salt)
Mixture: Combination of two or more substances (elements/compounds) in any ratio. Example: Air, seawater, salt solution
Differences:
1. Elements have one type of atom; compounds have fixed atom ratios; mixtures have variable ratios
2. Compounds have new properties; mixtures retain original properties
3. Compounds require chemical process to separate; mixtures use physical methods
Q (2023, 2 marks): What is the formula mass of H2SO4 (sulphuric acid)? (H=1, S=32, O=16)
Answer: Formula mass of H2SO4 = 2(1) + 1(32) + 4(16)
= 2 + 32 + 64
= 98 u
Q (2021, 3 marks): State Avogadro's law and explain its significance in stoichiometry.
Answer: Avogadro's Law: Equal volumes of gases at the same temperature and pressure contain equal number of molecules.
Mathematically: V ∝ n (at constant T and P)
Significance in stoichiometry:
1. Allows calculation of moles from volume of gases
2. Helps predict volume ratios in gaseous reactions
3. Avogadro's number (6.022 × 10^23) bridges microscopic and macroscopic scales
4. Essential for mole calculations in chemical reactions
Example: 1 mole of any gas occupies 22.4 L at STP
Q (2022, 3 marks): Calculate the number of molecules in 44 g of CO2. (C=12, O=16)
Answer: Molar mass of CO2 = 12 + 2(16) = 12 + 32 = 44 g/mol
Number of moles = Mass / Molar mass = 44 g / 44 g/mol = 1 mol
Number of molecules = 1 × 6.022 × 10^23 = 6.022 × 10^23 molecules
Q (2023, 3 marks): Explain the difference between molecular mass and molar mass. Can they be numerically equal?
Answer: Molecular mass: Sum of atomic masses of all atoms in a molecule (in atomic mass units, u)
Molar mass: Mass of one mole of substance (in grams/mol)
Numerically equal: Yes! The numerical values are equal but units differ.
Example for H2O:
Molecular mass = 2(1) + 16 = 18 u
Molar mass = 2(1) + 16 = 18 g/mol
Explanation: Since 1 mole = 6.022 × 10^23 particles, and 1 u = 1.66 × 10^-24 g, the conversion factor makes numerical values equal
Frequently Asked Questions
What is an atomic mass unit (u) and why is it used?
Atomic mass unit (u) is 1/12th the mass of a carbon-12 atom. It is used because atomic and molecular masses are extremely small; using u avoids very large decimal numbers. 1 u = 1.66 × 10^-24 g.
What is the mole and why is it important in chemistry?
A mole is the SI unit of amount of substance, defined as 6.022 × 10^23 particles (atoms, molecules, ions). It bridges the gap between atomic scale and macroscopic quantities, making calculations in chemistry possible.
More Class 9 Science PYQs
- Chemical Reactions and Equations
- Light (Reflection and Refraction)
- Electricity
- Acids, Bases and Salts
- Metals and Non-metals
- Life Processes
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