Atomic & Molecular Masses — NEET Chemistry MCQs with Solutions
Free NEET Chemistry Atomic & Molecular Masses MCQs with step-by-step solutions (7 questions). Part of Some Basic Concepts of Chemistry. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — Atomic & Molecular Masses · easy · theory
On the modern carbon-12 scale, one unified atomic mass unit (1 u) is defined as a mass exactly equal to:
A. the mass of one atom of carbon-12
B. the mass of one atom of hydrogen
C. one-twelfth of the mass of one atom of oxygen-16
D. one-twelfth of the mass of one atom of carbon-12 ✓ Correct
Solution: Since 1961 the atomic mass scale takes carbon-12 as the standard, assigning ¹²C a mass of exactly 12 u. One unified atomic mass unit (1 u, formerly amu) is therefore defined as exactly one-twelfth of the mass of one ¹²C atom, which equals 1.66056 × 10⁻²⁴ g. Hydrogen and oxygen-16 are not the reference on this scale.
Q2 — Atomic & Molecular Masses · easy · numerical
Using the atomic masses H = 1 u and O = 16 u, the molecular mass of water (H₂O) is:
A. 34 u
B. 16 u
C. 18 u ✓ Correct
D. 17 u
Solution: Molecular mass = sum of atomic masses = 2 × (mass of H) + 1 × (mass of O) = 2(1) + 16 = 18 u. Using only one hydrogen atom (1 + 16) wrongly gives 17 u; doubling the whole molecule gives 34 u; counting oxygen alone gives 16 u.
Q3 — Atomic & Molecular Masses · easy · numerical
The molecular mass of carbon dioxide (CO₂), taking C = 12 u and O = 16 u, is:
A. 60 u
B. 44 u ✓ Correct
C. 28 u
D. 22 u
Solution: CO₂ has one carbon and two oxygen atoms: molecular mass = 12 + 2 × 16 = 12 + 32 = 44 u. Counting only one oxygen (12 + 16) gives 28 u (that is CO); halving gives 22 u; using three oxygen atoms (12 + 48) gives 60 u.
Q4 — Atomic & Molecular Masses · easy · numerical
Sodium chloride does not exist as discrete molecules, so we use its formula mass. With Na = 23 u and Cl = 35.5 u, the formula mass of NaCl is:
A. 58.5 u ✓ Correct
B. 46 u
C. 58 u
D. 35.5 u
Solution: Formula mass of NaCl = atomic mass of sodium + atomic mass of chlorine = 23 + 35.5 = 58.5 u. Rounding chlorine to 35 gives 58 u; counting only chlorine gives 35.5 u; taking two sodium atoms (2 × 23) gives 46 u.
Q5 — Atomic & Molecular Masses · medium · numerical
Naturally occurring chlorine is a mixture of two isotopes, ³⁵Cl (abundance 75%) and ³⁷Cl (abundance 25%). Its average atomic mass is:
A. 35 u
B. 35.5 u ✓ Correct
C. 37 u
D. 36 u
Solution: Average atomic mass = weighted mean = (0.75 × 35) + (0.25 × 37) = 26.25 + 9.25 = 35.5 u. Taking a plain (unweighted) average of 35 and 37 wrongly gives 36 u. This weighted mean is why the tabulated atomic mass of chlorine is the fractional value 35.5 u.
Q6 — Atomic & Molecular Masses · medium · numerical
The molecular mass of calcium carbonate (CaCO₃), using Ca = 40 u, C = 12 u and O = 16 u, is:
A. 100 u ✓ Correct
B. 88 u
C. 84 u
D. 68 u
Solution: CaCO₃ contains one Ca, one C and three O atoms: 40 + 12 + (3 × 16) = 40 + 12 + 48 = 100 u. Forgetting the carbon (40 + 48) gives 88 u; using only one oxygen (40 + 12 + 16) gives 68 u; confusing Ca with Mg (24) gives 24 + 12 + 48 = 84 u.
Q7 — Atomic & Molecular Masses · easy · numerical
The molecular mass of ammonia (NH₃), taking N = 14 u and H = 1 u, is:
A. 14 u
B. 17 u ✓ Correct
C. 16 u
D. 15 u
Solution: NH₃ has one nitrogen and three hydrogen atoms: 14 + (3 × 1) = 14 + 3 = 17 u. Counting only one hydrogen gives 15 u; counting two hydrogens gives 16 u; counting nitrogen alone gives 14 u.