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Atoms, Molecules & Structure — Homi Bhabha Exam Chemistry MCQs with Solutions
Free Homi Bhabha Exam 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 · 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.
Q2 — 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.
Q3 — 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.
Q4 — 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.
Q5 — 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.
Q6 — 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 $= A - Z = 23 - 11 = 12$.
Q7 — 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).
Q8 — 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.
Q9 — 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 $37 - 17 = 20$.
Q10 — 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.
Q11 — 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.
Q12 — 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.
Q13 — 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 $= 2 \times 6.022 \times 10^{23} = 1.2044 \times 10^{24}$.
Q14 — 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.
Q15 — 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.
Q16 — 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.
Q17 — 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.
Q18 — Dalton's Atomic Theory · medium · theory
Dalton's theory explains that atoms combine to form compounds in:
A. random fractional ratios
B. small whole-number ratios ✓ Correct
C. ratios that keep changing
D. a 1:1 ratio only
Solution: Atoms of different elements combine in simple whole-number ratios to form compounds.
Q19 — Atomic Number & Mass Number · medium · theory
The atomic number of an element is equal to the number of ______ in its atom:
A. protons ✓ Correct
B. electrons and neutrons
C. neutrons
D. nucleons
Solution: Atomic number (Z) = number of protons in the nucleus (equal to electrons in a neutral atom).
Q20 — Atomic Number & Mass Number · medium · theory
The mass number of an atom is the total number of:
A. protons only
B. protons and neutrons ✓ Correct
C. protons and electrons
D. neutrons and electrons
Solution: Mass number (A) = number of protons + number of neutrons (the nucleons).
Q21 — Atomic Number & Mass Number · medium · theory
The identity of an element is decided by its:
A. mass number
B. valency
C. atomic number ✓ Correct
D. number of neutrons
Solution: The number of protons (atomic number) uniquely determines which element an atom is.
Q22 — Isotopes · medium · theory
Isotopes of an element have the same number of ______ but a different number of ______:
A. electrons; protons
B. neutrons; protons
C. protons; neutrons ✓ Correct
D. protons; electrons
Solution: Isotopes have the same atomic number (protons) but different mass numbers (neutrons).
Q23 — Isotopes · medium · theory
Carbon-12 and Carbon-14 are isotopes because they have the same number of protons but different numbers of:
A. electrons
B. protons
C. molecules
D. neutrons ✓ Correct
Solution: Both have 6 protons; C-12 has 6 neutrons and C-14 has 8 neutrons.
Q24 — Isotopes · medium · theory
The isotope used for carbon dating of ancient fossils is:
A. Carbon-14 ✓ Correct
B. Cobalt-60
C. Carbon-12
D. Uranium-235
Solution: The radioactive isotope Carbon-14 is used to estimate the age of once-living materials.
Q25 — Valency · medium · theory
The combining capacity of an element is called its:
A. mass number
B. atomic number
C. atomicity
D. valency ✓ Correct
Solution: Valency is the number of bonds an atom can form, i.e. its combining capacity.
Q26 — Valency · medium · theory
The valency of oxygen in most of its compounds is:
A. 2 ✓ Correct
B. 1
C. 3
D. 4
Solution: Oxygen typically forms two bonds, so its valency is 2 (e.g. H₂O).
Q27 — Valency · medium · theory
The valency of carbon, which allows it to form four bonds, is:
A. 4 ✓ Correct
B. 1
C. 3
D. 2
Solution: Carbon has 4 electrons in its outer shell and a valency of 4 (tetravalent).
Q28 — Mole Concept · medium · theory
One mole of any substance contains a number of particles equal to:
A. 1.6 × 10⁻¹⁹
B. 9.8
C. 6.022 × 10²³ (Avogadro number) ✓ Correct
D. 3.0 × 10⁸
Solution: One mole contains Avogadro's number, $6.022 \times 10^{23}$ particles.
Q29 — Mole Concept · medium · numerical
The number of moles in 36 g of water (molar mass 18 g/mol) is:
A. 36 mol
B. 18 mol
C. 2 mol ✓ Correct
D. 0.5 mol
Solution: Moles $= \dfrac{\text{mass}}{\text{molar mass}} = \dfrac{36}{18} = 2$ mol.
Q30 — Mole Concept · medium · numerical
The number of moles in 44 g of carbon dioxide (molar mass 44 g/mol) is:
A. 1 mol ✓ Correct
B. 0.5 mol
C. 44 mol
D. 2 mol
Solution: Moles $= \dfrac{44}{44} = 1$ mol.