Atoms and Molecules (Class 9 Science)
Atoms are the smallest indivisible particles of matter, while molecules are formed when two or more atoms combine chemically. This chapter explores the fun
TL;DR: Atoms are the smallest indivisible particles of matter, while molecules are formed when two or more atoms combine chemically. This chapter explores th…
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Atoms are the smallest indivisible particles of matter, while molecules are formed when two or more atoms combine chemically. This chapter explores the fun
Atoms and Molecules MCQs with Answers & Explanations
Q1. What is the atomic mass unit (amu) approximately equal to?
- Mass of one proton
- 1/12th of carbon-12 atom ✓ (correct)
- Mass of one electron
- 1/16th of oxygen atom
Explanation: The atomic mass unit is defined as 1/12th of the mass of a carbon-12 atom, making it the standard for measuring atomic masses.
Q2. Which of the following has the smallest mass?
- Proton
- Neutron
- Electron ✓ (correct)
- Alpha particle
Explanation: An electron has a mass of approximately 9.11 × 10^-31 kg, which is much smaller than a proton or neutron.
Q3. What is Avogadro's number?
- 6.02 × 10^22
- 6.02 × 10^21
- 6.02 × 10^24
- 6.02 × 10^23 ✓ (correct)
Explanation: Avogadro's number (6.022 × 10^23) represents the number of particles in one mole of any substance.
Q4. The molecular mass of CO2 is (C=12, O=16):
- 44 ✓ (correct)
- 36
- 32
- 28
Explanation: Molecular mass of CO2 = 12 + 16×2 = 12 + 32 = 44 u.
Q5. How many atoms are present in one molecule of H2SO4?
- 8
- 7 ✓ (correct)
- 5
- 6
Explanation: H2SO4 contains 2 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms, totaling 7 atoms per molecule.
Q6. Which compound has the empirical formula CH and molecular mass 78?
- C4H4
- C5H5
- C6H6 ✓ (correct)
- C3H3
Explanation: If empirical formula is CH (mass 13) and molecular mass is 78, then n = 78/13 = 6. So molecular formula is C6H6.
Q7. The mass of 0.5 mole of oxygen (O2) is (O=16):
- 8 g
- 64 g
- 32 g
- 16 g ✓ (correct)
Explanation: Mass = moles × molar mass = 0.5 × 32 = 16 g. (Molar mass of O2 = 16×2 = 32 g/mol)
Q8. Number of molecules in 11.2 L of CO2 at STP is:
- 3.011 × 10^23 ✓ (correct)
- 1.505 × 10^23
- 9.033 × 10^23
- 6.022 × 10^23
Explanation: At STP one mole occupies 22.4 L, so 11.2 L is 0.5 mol. Molecules = 0.5 x 6.022 x 10²³ = 3.011 x 10²³.
Q9. Which of the following has the highest molecular mass?
- NH3
- CO2 ✓ (correct)
- H2O
- CH4
Explanation: CO2 has molecular mass 44, H2O has 18, NH3 has 17, CH4 has 16. So CO2 has the highest mass.
Q10. The number of atoms in 2 moles of chlorine (Cl2) is:
- 6.022 × 10^23
- 3.011 × 10^23
- 2.411 × 10^24 ✓ (correct)
- 12.044 × 10^23
Explanation: 2 moles of Cl2 contains 2 × 2 = 4 moles of Cl atoms. Number of atoms = 4 × 6.022 × 10^23 = 2.409 × 10^24 ≈ 2.411 × 10^24.
Q11. Who proposed the Dalton's atomic theory?
- Thomson
- Bohr
- Rutherford
- John Dalton ✓ (correct)
Explanation: John Dalton proposed the atomic theory in 1808, stating that matter consists of tiny indivisible particles called atoms.
Q12. The mass number of an atom is equal to:
- Number of protons + neutrons ✓ (correct)
- Number of protons
- Number of electrons + neutrons
- Number of electrons
Explanation: The mass number is the sum of protons and neutrons, which constitute most of the atom's mass.
Q13. Which of the following is the correct electronic configuration of Oxygen (O)?
- 8, 2
- 2, 6 ✓ (correct)
- 2, 4
- 2, 8
Explanation: Oxygen has atomic number 8, so its electronic configuration is 2, 6 (2 electrons in first shell, 6 in second).
Q14. The molecular mass of CO2 is approximately:
- 32
- 64
- 44 ✓ (correct)
- 28
Explanation: Molecular mass of CO2 = 12 + (16 x 2) = 12 + 32 = 44 g/mol.
Q15. An ion with a positive charge is called:
- Atom
- Anion
- Radical
- Cation ✓ (correct)
Explanation: A cation is an ion with a positive charge, formed when an atom loses electrons.
Q16. The atomic mass unit is defined as:
- 1/12 of carbon-12 mass ✓ (correct)
- Mass of proton
- Mass of electron
- 1/16 of oxygen mass
Explanation: One atomic mass unit (u) is defined as 1/12 of the mass of a carbon-12 atom.
Q17. The formula for common salt is:
- NaCl2
- NaCl ✓ (correct)
- Na2Cl
- Na2Cl2
Explanation: The formula for sodium chloride (common salt) is NaCl, with one sodium and one chloride ion.
Q18. Which of the following is a diatomic molecule?
- P4
- O3
- H2 ✓ (correct)
- S8
Explanation: H2 (hydrogen) is a diatomic molecule consisting of two hydrogen atoms bonded together.
Q19. The valency of oxygen in most compounds is:
- 3
- 4
- 1
- 2 ✓ (correct)
Explanation: Oxygen has a valency of 2 in most compounds because it needs 2 more electrons to complete its octet.
Q20. The molar mass of H2SO4 is approximately:
- 98 ✓ (correct)
- 100
- 96
- 102
Explanation: Molar mass of H2SO4 = (2 x 1) + 32 + (4 x 16) = 2 + 32 + 64 = 98 g/mol.
Frequently Asked Questions
What is the difference between atomic mass and atomic number?
Atomic number is the number of protons in an atom and determines the element's identity. Atomic mass is the sum of protons and neutrons (mass number) and determines the isotope.
Why is the mole concept important in chemistry?
The mole concept provides a bridge between the macroscopic world (grams we can measure) and the microscopic world (atoms and molecules), making calculations involving large numbers of particles manageable.
More Class 9 Science MCQs
- Light Reflection and Refraction
- Electricity
- Acids Bases and Salts
- Life Processes
- Carbon and its Compounds
- Metals and Non-metals
🤖 Stuck on any of these? Ask Syllab's free AI Tutor to explain step by step →