Dalton's Atomic Theory — IISER Chemistry MCQs with Solutions
Free IISER Chemistry Dalton's Atomic Theory MCQs with step-by-step solutions (7 questions). Part of Some Basic Concepts of Chemistry. Practise online on Prepizo — no login needed.
▶ Practise Dalton's Atomic Theory online (free)
Questions with solutions
Q1 — Dalton's Atomic Theory · medium · theory
Assertion (A): Dalton’s atomic theory successfully explained the law of conservation of mass. Reason (R): According to Dalton, atoms are neither created nor destroyed in a chemical reaction but are merely rearranged.
A. Both A and R are true, but R is not the correct explanation of A
B. A is false, but R is true
C. A is true, but R is false
D. Both A and R are true, and R is the correct explanation of A ✓ Correct
Solution: Both statements are true. Because atoms are only rearranged and neither created nor destroyed (R), the total mass of atoms — and hence of matter — is unchanged in a reaction, which is exactly the law of conservation of mass (A). So R is the correct explanation of A.
Q2 — Dalton's Atomic Theory · hard · theory
Assertion (A): Dalton’s atomic theory could not explain Gay Lussac’s law of combining gaseous volumes. Reason (R): Dalton believed that atoms of the same element could not combine, so molecules such as H₂ and O₂ did not exist.
A. A is true, but R is false
B. A is false, but R is true
C. Both A and R are true, and R is the correct explanation of A ✓ Correct
D. Both A and R are true, but R is not the correct explanation of A
Solution: Both are true. Because Dalton rejected molecules made of like atoms (R), he could not accept that two volumes of hydrogen combine with one volume of oxygen to give two volumes of water vapour; only Avogadro’s polyatomic molecules explained this. Hence R is the correct reason that A holds.
Q3 — Dalton's Atomic Theory · hard · theory
Assertion (A): The discovery of isotopes did not force scientists to abandon Dalton’s atomic theory entirely. Reason (R): Only the postulate that all atoms of an element are identical in mass had to be modified, while the central idea that atoms are the units of chemical combination was retained.
A. A is true, but R is false
B. Both A and R are true, and R is the correct explanation of A ✓ Correct
C. A is false, but R is true
D. Both A and R are true, but R is not the correct explanation of A
Solution: Both are true. Isotopes clash only with the "identical mass" postulate; the rest of Dalton’s framework — atoms as the building blocks that combine in fixed ratios — survives. Because the theory needed modification rather than rejection (R), it was not discarded (A), so R correctly explains A.
Q4 — Dalton's Atomic Theory · hard · theory
Despite its later-recognised limitations, Dalton’s atomic theory is regarded as a landmark in chemistry mainly because it:
A. correctly described the internal structure of the atom
B. predicted the existence of isotopes and isobars
C. gave a quantitative theory of the chemical bond
D. provided a single theoretical model that accounted for the known laws of chemical combination ✓ Correct
Solution: Dalton’s achievement was to unify the empirical laws of chemical combination (conservation of mass, definite and multiple proportions) under one atomic model. It said nothing correct about atomic structure, bonding, isotopes or isobars — those understandings came much later.
Q5 — Dalton's Atomic Theory · hard · theory
A sample of carbon dioxide obtained from burning coal and another obtained from fermentation are both found to contain carbon and oxygen in the same 3:8 mass ratio. Which postulate of Dalton’s theory most directly accounts for this?
A. Atoms are indivisible
B. All atoms of an element are identical in mass
C. Atoms are rearranged in chemical reactions
D. Compounds are formed when atoms of different elements combine in a fixed ratio ✓ Correct
Solution: If CO₂ is always built from carbon and oxygen atoms in the same fixed ratio (1 C : 2 O, i.e. 12:32 = 3:8 by mass), then every pure sample must show that same composition regardless of source. This "fixed ratio" postulate is the atomic-theory basis of the law of definite proportions.
Q6 — Dalton's Atomic Theory · medium · theory
Which statement about the historical development of atomic ideas is correct?
A. Democritus proposed the atomic idea only after reading Dalton’s work
B. Dalton was the first person in history to suggest that matter is made of atoms
C. Acharya Kanad (~600 BCE) called the smallest particle "Paramanu" and Democritus called it "a-tomio", both long before Dalton gave the idea an experimental, quantitative basis in 1808 ✓ Correct
D. Acharya Kanad is credited with introducing the mole concept
Solution: The notion of indivisible particles is ancient: Acharya Kanad (~600 BCE) proposed "Paramanu" and the Greek philosopher Democritus (460–370 BC) spoke of "a-tomio" (indivisible). Dalton’s contribution in 1808 was to place this old philosophical idea on a firm experimental and quantitative footing.
Q7 — Dalton's Atomic Theory · hard · numerical
An element X forms two oxides. The first contains 20.0% oxygen by mass and the second contains 33.3% oxygen by mass. For a fixed mass of X, the ratio of the mass of oxygen in the second oxide to that in the first is:
A. 1:2
B. 2:1 ✓ Correct
C. 5:3
D. 3:5
Solution: For a fixed mass of X, oxygen combined per unit X = (%O)/(%X). First oxide: 20/80 = 0.25; second oxide: 33.3/66.7 = 0.50. Ratio (second:first) = 0.50:0.25 = 2:1, a simple whole-number ratio consistent with the law of multiple proportions. Comparing the oxygen percentages directly (33.3:20 ≈ 5:3) is wrong because it ignores that the mass of X must be held constant.