Practice MCQs for Class 9 Chemistry Atoms and Molecules
Review structured MCQ sets for Class 9 Chemistry Atoms and Molecules. Built according to official CBSE guidelines, these downloadable questions support daily revision and core concept reinforcement.
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Multiple Choice Questions
(a) half a gram
(b) 2 g
(c) 1 g
(d) 4 g
Show Answer & Explanation
Answer: (c) 1 g
Explanation:
1. Avogadro's number of atoms is one mole of atoms, which is one gram atom.
2. The atomic mass of hydrogen is 1 u, so one gram atom of hydrogen weighs 1 g.
Teacher's Note:
1. One mole of atoms weighs as many grams as the atomic mass.
2. Hydrogen atomic mass is about 1.
(a) 1
(b) 2
(c) 3
(d) 4
Show Answer & Explanation
Answer: (d) 4
Explanation:
1. Gram atoms = mass / atomic mass = 128/32 = 4.
Teacher's Note:
1. Atomic mass of sulphur is 32.
2. Divide the given mass by 32.
(a) 145
(b) 245
(c) 345
(d) 100
Show Answer & Explanation
Answer: (b) 245
Explanation:
1. Molar mass of KClO3 = 39 + 35.5 + 3 × 16 = 122.5 g.
2. Mass = 2 × 122.5 = 245 g.
Teacher's Note:
1. Mass = moles × molar mass.
2. Add up the atomic masses of K, Cl and three O atoms.
(a) 1
(b) 2
(c) 3
(d) 4
Show Answer & Explanation
Answer: (d) 4
Explanation:
1. Molar mass of CaCO3 = 40 + 12 + 48 = 100 g.
2. Moles = 400/100 = 4.
Teacher's Note:
1. Moles = mass / molar mass.
2. CaCO3 has molar mass 100 g.
(a) \( 12\times10^{22} \)
(b) \( 12\times10^{24} \)
(c) \( 12\times10^{23} \)
(d) \( 12\times10^{21} \)
Show Answer & Explanation
Answer: (c) \( 12\times10^{23} \)
Explanation:
1. Molar mass of CH4 = 16 g, so 3.2 g is 0.2 mol.
2. One CH4 molecule has 6 + 4 = 10 electrons, so 0.2 mol has 2 mol of electrons.
3. \( 2\times6.02\times10^{23}\approx12\times10^{23} \).
Teacher's Note:
1. Count electrons in one molecule first.
2. Multiply by the number of molecules.
(a) 24 g of C (atomic mass 12)
(b) 56 g of Fe (atomic mass 56)
(c) 27 g of Al (atomic mass 27)
(d) 108 g of Ag (atomic mass 108)
Show Answer & Explanation
Answer: (a) 24 g of C (atomic mass 12)
Explanation:
1. Moles of C = 24/12 = 2; Fe = 56/56 = 1; Al = 27/27 = 1; Ag = 108/108 = 1.
2. The number of atoms is proportional to moles, so 24 g of C has the most.
Teacher's Note:
1. Convert each mass to moles.
2. More moles means more atoms.
(a) \( 2.1\times10^{20} \)
(b) \( 0.21\times10^{20} \)
(c) \( 210\times10^{20} \)
(d) \( 21\times10^{20} \)
Show Answer & Explanation
Answer: (d) \( 21\times10^{20} \)
Explanation:
1. 7 mg = 0.007 g, which is 0.007/14 = 5 × 10^-4 mol of atoms.
2. Each N-14 atom has 14 − 7 = 7 neutrons.
3. \( 5\times10^{-4}\times7\times6.02\times10^{23}\approx2.1\times10^{21}=21\times10^{20} \).
Teacher's Note:
1. Neutrons per atom = mass number − atomic number.
2. Number of atoms = moles × Avogadro number.
(a) \( 24\times10^{20} \)
(b) \( 24\times10^{21} \)
(c) \( 24\times10^{22} \)
(d) \( 24\times10^{19} \)
Show Answer & Explanation
Answer: (a) \( 24\times10^{20} \)
Explanation:
1. 7 mg = 0.007 g = 0.007/14 = 5 × 10^-4 mol of atoms.
2. Each C-14 atom has 14 − 6 = 8 neutrons.
3. Neutrons = 5×10^-4 × 8 × 6.02×10^23 ≈ 2.4×10^21, each of about 1 amu, so the mass is about 24×10^20 amu.
Teacher's Note:
1. Find the number of neutrons, then multiply by the mass of one neutron (1 amu).
2. Neutrons per atom = 14 − 6.
(a) 32
(b) 64
(c) 60
(d) 62
Show Answer & Explanation
Answer: (a) 32
Explanation:
1. Neutral CO3 has 6 + 3 × 8 = 30 electrons.
2. The 2− charge adds 2 electrons, giving 32.
Teacher's Note:
1. Add the atomic numbers of all atoms.
2. A negative charge adds electrons.
(a) 5.6 L
(b) 0.56 L
(c) 0.0056 L
(d) 56 L
Show Answer & Explanation
Answer: (d) 56 L
Explanation:
1. One mole of any gas occupies 22.4 L at NTP.
2. 2.5 × 22.4 = 56 L.
Teacher's Note:
1. Volume = moles × 22.4 L.
2. Check the decimal place.
(a) 0.112 L
(b) 1.12 L
(c) 11.2 L
(d) 112 L
Show Answer & Explanation
Answer: (c) 11.2 L
Explanation:
1. 0.5 × 22.4 = 11.2 L.
Teacher's Note:
1. Half a mole occupies half of 22.4 L.
(a) 67.2 L
(b) 44.8 L
(c) 22.4 L
(d) 89.6 L
Show Answer & Explanation
Answer: (a) 67.2 L
Explanation:
1. The reaction is 2BCl3 + 3H2 → 2B + 6HCl.
2. 21.6 g of B is 21.6/10.8 = 2 mol, which needs 3 mol of H2.
3. 3 × 22.4 = 67.2 L.
Teacher's Note:
1. Balance the equation first.
2. 273 K and 1 atm is the same as STP.
(a) 1.25 and \( 75.2\times10^{23} \)
(b) 1.25 and \( 7.52\times10^{23} \)
(c) 12.5 and \( 7.52\times10^{23} \)
(d) 0.125 and \( 7.52\times10^{23} \)
Show Answer & Explanation
Answer: (b) 1.25 and \( 7.52\times10^{23} \)
Explanation:
1. Moles of O2 = 40/32 = 1.25.
2. Molecules = 1.25 × 6.02×10^23 = 7.52×10^23.
Teacher's Note:
1. Molar mass of O2 is 32 g.
2. Molecules = moles × Avogadro number.
(a) 4.8
(b) 48
(c) 0.48
(d) 480
Show Answer & Explanation
Answer: (a) 4.8
Explanation:
1. Mass = 0.4 × 12 = 4.8 g.
Teacher's Note:
1. One gram atom of carbon is 12 g.
(a) 687
(b) 6.87
(c) 68.7
(d) 0.687
Show Answer & Explanation
Answer: (b) 6.87
Explanation:
1. Mass of the compound = 42.6% of 5000 g = 2130 g.
2. Molar mass of Ca3(PO4)2 = 120 + 62 + 128 = 310 g.
3. Moles = 2130/310 = 6.87.
Teacher's Note:
1. Find the mass of the compound first.
2. Ca3(PO4)2 has 3 Ca, 2 P and 8 O.
(a) 36 g, 96 g
(b) 96 g, 36 g
(c) 3.6 g, 9.6 g
(d) 9.6 g, 3.6 g
Show Answer & Explanation
Answer: (a) 36 g, 96 g
Explanation:
1. 3 mol of CO2 contains 3 mol of C and 6 mol of O atoms.
2. Carbon: 3 × 12 = 36 g. Oxygen: 6 × 16 = 96 g.
Teacher's Note:
1. One CO2 has one C atom and two O atoms.
(a) 0.56 L
(b) 5.6 L
(c) 56 L
(d) 560 L
Show Answer & Explanation
Answer: (a) 0.56 L
Explanation:
1. Moles = \( 1.5\times10^{22}/6.02\times10^{23}\approx0.0249 \).
2. Volume = 0.0249 × 22.4 ≈ 0.56 L.
Teacher's Note:
1. Convert molecules to moles first.
(a) \( 1.25\times10^{-2} \)
(b) \( 2.5\times10^{-2} \)
(c) 0.02
(d) \( 3.125\times10^{-2} \)
Show Answer & Explanation
Answer: (d) \( 3.125\times10^{-2} \)
Explanation:
1. One formula unit of Mg3(PO4)2 has 8 oxygen atoms.
2. Moles = 0.25/8 = 0.03125 = 3.125×10^-2.
Teacher's Note:
1. Count the oxygen atoms per formula unit.
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Practice MCQs for Class 9 Chemistry Atoms and Molecules
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FAQs
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