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Atoms, Molecules & Structure — Homi Bhabha Class 9 Chemistry MCQs with Solutions
Free Homi Bhabha Class 9 Chemistry Atoms, Molecules & Structure MCQs with step-by-step solutions covering Laws of Chemical Combination, Dalton's Atomic Theory, Atomic Number & Mass Number, Isotopes, Valency, Mole Concept. Practise online on Prepizo — no login needed.
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Sample questions with solutions
Q1 — Laws of Chemical Combination · easy · theory
The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. Who first established this law through careful experiments?
A. Antoine Lavoisier ✓ Correct
B. J.J. Thomson
C. John Dalton
D. Joseph Proust
Solution: Antoine Lavoisier established the law of conservation of mass by weighing reactants and products in closed systems.
Q2 — Dalton's Atomic Theory · easy · theory
According to Dalton's atomic theory, atoms are:
A. Indivisible and indestructible ✓ Correct
B. Able to be created and destroyed in reactions
C. Made of protons, neutrons and electrons
D. Identical in mass for every element
Solution: Dalton postulated that atoms are the smallest indivisible particles of matter that cannot be created or destroyed.
Q3 — Atomic Number & Mass Number · easy · theory
The atomic number of an element is equal to the number of:
A. Electrons plus neutrons
B. Protons plus neutrons
C. Protons in the nucleus ✓ Correct
D. Neutrons in the nucleus
Solution: The atomic number (Z) is defined as the number of protons in the nucleus of an atom.
Q4 — Isotopes · easy · theory
Isotopes are atoms of the same element that have:
A. The same mass number but different atomic numbers
B. Different numbers of both protons and electrons
C. The same number of neutrons but different numbers of protons
D. The same number of protons but different numbers of neutrons ✓ Correct
Solution: Isotopes share the same atomic number (protons) but differ in the number of neutrons, giving them different mass numbers.
Q5 — Valency · easy · theory
The valency of an element is best described as:
A. The number of shells in its atom
B. Its atomic mass
C. The number of neutrons in its atom
D. Its combining capacity ✓ Correct
Solution: Valency is the combining capacity of an element, decided by the number of electrons it gains, loses or shares to become stable.
Q6 — Mole Concept · easy · theory
One mole of any substance contains the same number of particles, known as Avogadro's number. This number is:
A. 6.022 x 10²²
B. 6.022 x 10²³ ✓ Correct
C. 3.011 x 10²³
D. 1.66 x 10⁻²⁴
Solution: One mole of any substance contains Avogadro's number of particles, which is 6.022 x 10²³.
Q7 — Laws of Chemical Combination · hard · theory
The law of constant (definite) proportions states that a pure chemical compound always contains the same elements combined in a:
A. ratio of 1:1 always
B. variable ratio by mass
C. fixed ratio by mass ✓ Correct
D. ratio that depends on temperature
Solution: A given compound always has its constituent elements in the same fixed mass ratio.
Q8 — Laws of Chemical Combination · hard · numerical
In water, hydrogen and oxygen are always combined in the mass ratio 1 : 8. The mass of oxygen that combines with 3 g of hydrogen is:
A. 3 g
B. 24 g ✓ Correct
C. 11 g
D. 8 g
Solution: Ratio 1 : 8 means $3 \times 8 = 24$ g of oxygen combines with 3 g of hydrogen.
Q9 — Laws of Chemical Combination · hard · numerical
When 12 g of carbon burns completely in 32 g of oxygen, the mass of carbon dioxide formed (by conservation of mass) is:
A. 20 g
B. 32 g
C. 384 g
D. 44 g ✓ Correct
Solution: Mass of products = mass of reactants $= 12 + 32 = 44$ g.
Q10 — Dalton's Atomic Theory · hard · theory
Which postulate of Dalton's atomic theory was later found to be incorrect?
A. Matter is made up of atoms
B. Atoms combine in small whole-number ratios
C. Atoms of different elements differ
D. Atoms are indivisible ✓ Correct
Solution: Atoms are actually divisible into protons, neutrons and electrons, so this postulate is wrong.
Q11 — Dalton's Atomic Theory · hard · theory
The idea that during a chemical reaction atoms are neither created nor destroyed but only rearranged is consistent with:
A. the discovery of isotopes
B. the mole concept
C. the law of constant proportion only
D. the law of conservation of mass ✓ Correct
Solution: Since atoms are merely rearranged, the total mass is conserved.
Q12 — Atomic Number & Mass Number · hard · numerical
An atom of sodium has atomic number 11 and mass number 23. The number of neutrons in it is:
A. 12 ✓ Correct
B. 11
C. 34
D. 23
Solution: Neutrons $= A - Z = 23 - 11 = 12$.
Q13 — Atomic Number & Mass Number · hard · numerical
An atom has 17 protons and 18 neutrons. Its mass number is:
A. 35 ✓ Correct
B. 17
C. 18
D. 1
Solution: Mass number $= 17 + 18 = 35$ (this is chlorine).
Q14 — Isotopes · hard · theory
Isotopes of an element show ______ chemical properties because they have the same ______:
A. identical; number of electrons (electronic configuration) ✓ Correct
B. different; number of neutrons
C. identical; number of neutrons
D. different; atomic number
Solution: Chemical behaviour depends on electrons; isotopes have the same electron arrangement, so identical chemistry.
Q15 — Isotopes · hard · numerical
Two isotopes of an element have mass numbers 35 and 37 with atomic number 17. The numbers of neutrons in them are:
A. 18 and 20 ✓ Correct
B. 18 and 18
C. 35 and 37
D. 17 and 17
Solution: Neutrons $= A - Z$: $35 - 17 = 18$ and $37 - 17 = 20$.
Q16 — Valency · hard · numerical
Aluminium has a valency of 3 and oxygen a valency of 2. The formula of aluminium oxide is:
A. AlO₂
B. Al₂O₃ ✓ Correct
C. AlO
D. Al₃O₂
Solution: Cross-multiplying the valencies (Al = 3, O = 2) gives Al₂O₃.
Q17 — Valency · hard · numerical
Calcium (valency 2) combines with chlorine (valency 1). The formula of calcium chloride is:
A. Ca₂Cl₃
B. CaCl
C. Ca₂Cl
D. CaCl₂ ✓ Correct
Solution: One Ca (valency 2) balances two Cl (valency 1 each), giving CaCl₂.
Q18 — Mole Concept · hard · numerical
The mass of 0.5 mole of carbon dioxide (molar mass 44 g/mol) is:
A. 0.5 g
B. 22 g ✓ Correct
C. 88 g
D. 44 g
Solution: Mass $= \text{moles} \times \text{molar mass} = 0.5 \times 44 = 22$ g.
Q19 — Mole Concept · hard · numerical
The number of atoms present in 2 moles of a monatomic gas is:
A. 6.022 × 10²³
B. 1.2044 × 10²⁴ ✓ Correct
C. 2 × 10²³
D. 3.011 × 10²³
Solution: Atoms $= 2 \times 6.022 \times 10^{23} = 1.2044 \times 10^{24}$.
Q20 — Laws of Chemical Combination · hard · theory
Assertion: Pure water collected from a river and pure water prepared in a laboratory both contain hydrogen and oxygen in exactly the same ratio by mass. Reason: This is explained by the law of multiple proportions. Which is correct?
A. Both assertion and reason are true, and the reason correctly explains the assertion
B. Both the assertion and the reason are false
C. The assertion is true but the reason is false ✓ Correct
D. The assertion is false but the reason is true
Solution: A pure compound always has the same composition by the law of definite (constant) proportions, not multiple proportions. So the assertion is true but the stated reason is false.
Q21 — Dalton's Atomic Theory · hard · theory
The discovery of which of the following is a direct contradiction of one of Dalton's original postulates?
A. The law of definite proportions
B. Isotopes, which are atoms of the same element with different masses ✓ Correct
C. The law of conservation of mass
D. Elements combining in whole-number ratios
Solution: Dalton stated that all atoms of a given element are identical in mass. Isotopes are atoms of the same element with different masses, contradicting this postulate.
Q22 — Atomic Number & Mass Number · hard · numerical
An ion contains 17 protons, 18 neutrons and 18 electrons. What are its mass number and charge, respectively?
A. 35 and +1
B. 35 and -1 ✓ Correct
C. 53 and -1
D. 18 and -1
Solution: Mass number = protons + neutrons = 17 + 18 = 35. Charge = protons - electrons = 17 - 18 = -1, so it is a -1 ion.
Q23 — Isotopes · hard · numerical
Chlorine consists of Cl-35 and Cl-37 with relative abundances of 75% and 25% respectively. Its average atomic mass is closest to:
A. 35.5 u ✓ Correct
B. 36.0 u
C. 37.0 u
D. 35.0 u
Solution: Average atomic mass = (35 x 0.75) + (37 x 0.25) = 26.25 + 9.25 = 35.5 u.
Q24 — Valency · hard · theory
Aluminium has a valency of 3 and the sulphate group (SO4) has a valency of 2. Using the criss-cross method, the correct formula of aluminium sulphate is:
A. Al3(SO4)2
B. Al2(SO4)3 ✓ Correct
C. Al(SO4)2
D. AlSO4
Solution: Crossing the valencies, aluminium (3) and sulphate (2) exchange numbers to give Al2(SO4)3, which is electrically neutral.
Q25 — Mole Concept · hard · numerical
How many molecules are present in 8 g of methane, CH4? (Molar mass of CH4 = 16 g/mol)
A. 3.011 x 10²²
B. 6.022 x 10²³
C. 3.011 x 10²³ ✓ Correct
D. 1.204 x 10²⁴
Solution: Moles = 8 / 16 = 0.5 mol. Molecules = 0.5 x 6.022 x 10²³ = 3.011 x 10²³.
Q26 — Mole Concept · hard · numerical
What is the mass of 0.25 mole of carbon dioxide, CO2? (Molar mass of CO2 = 44 g/mol)
A. 176 g
B. 11 g ✓ Correct
C. 44 g
D. 22 g
Solution: Mass = number of moles x molar mass = 0.25 x 44 = 11 g.
Q27 — Laws of Chemical Combination · medium · theory
The law of conservation of mass states that in a chemical reaction, mass is:
A. neither created nor destroyed ✓ Correct
B. converted into energy
C. always decreased
D. always increased
Solution: The total mass of the reactants equals the total mass of the products.
Q28 — Laws of Chemical Combination · medium · theory
The law of constant proportions was proposed by the scientist:
A. Newton
B. Rutherford
C. Bohr
D. Proust ✓ Correct
Solution: Joseph Proust put forward the law of definite (constant) proportions.
Q29 — Dalton's Atomic Theory · medium · theory
According to Dalton's atomic theory, the smallest particle of matter that takes part in a chemical reaction is the:
A. atom ✓ Correct
B. proton
C. electron
D. molecule
Solution: Dalton proposed that atoms are the smallest indivisible particles taking part in chemical reactions.
Q30 — Dalton's Atomic Theory · medium · theory
According to Dalton, atoms of the same element are:
A. made of molecules
B. identical in mass and properties ✓ Correct
C. divisible into protons
D. always different
Solution: Dalton stated that all atoms of a given element are alike in mass and chemical properties.