Atomic Molecular Masses

8 MCQs2 revision cards9-step worked example
Source: NCERT Some Basic Concepts of ChemistryOfficial key: NTA-verifiedLast updated: 26 Sep 2026

Atomic Molecular Masses, explained for NEET

The number you look up on the periodic table is not a mass — it's a ratio. That single confusion costs marks in questions that look trivially easy.

Atomic mass unit (amu / u). One unified atomic mass unit is defined as exactly 1/12 the mass of one carbon-12 atom. In grams: 1 u = 1.66054 × 10⁻²⁴ g (NCERT Class 11 Chemistry Chapter 1, page 17). Carbon-12 is the reference standard — not hydrogen, not oxygen. NEET has tested whether students remember the reference isotope.

Atomic mass vs mass number. Mass number (A) is the integer count of protons + neutrons. Atomic mass is the weighted average over all naturally occurring isotopes and is almost never an integer. Chlorine's mass number for ³⁵Cl is 35, but its atomic mass is 35.5 u because nature provides roughly 75% ³⁵Cl and 25% ³⁷Cl. When a question gives "atomic mass of Cl = 35.5 u," that 35.5 already encodes the isotopic mix — do not round it to 35 or 36.

Molecular mass. Sum of atomic masses of every atom in the molecular formula. For H₂SO₄: 2(1.008) + 32.06 + 4(16.00) = 98.08 u. The operation is pure addition after multiplying each atomic mass by its subscript count. Errors creep in when students miscount atoms (especially oxygen in polyatomic ions) or confuse molecular mass (single molecule, in u) with molar mass (one mole, in g/mol). Numerically they are equal, but the units differ.

Formula mass. Ionic compounds (NaCl, CaCO₃) don't form discrete molecules, so we say "formula mass" instead of "molecular mass." The calculation is identical — sum of atomic masses per formula unit.

Watch out: Questions may ask "molecular mass of NaCl." Strictly, NaCl has a formula mass, not a molecular mass. NEET sometimes tests whether you flag this distinction or silently compute. Compute either way — but know the terminology difference if a conceptual option appears.

Can you answer these Atomic Molecular Masses MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

One unified atomic mass unit (1 u) is defined as:

Show answer and why every option is right or wrong

Answer: B. By IUPAC definition, 1 u = (1/12) × mass of one ¹²C atom (NCERT Class 11 Chemistry Chapter 1, page 16).

Why A is wrong: A is wrong because the oxygen-16 standard (1/16) was the older chemistry scale, replaced by the carbon-12 standard in 1961.

Why C is wrong: C is wrong because the hydrogen standard was the earliest proposal but was abandoned; hydrogen's atomic mass is 1.008 u, not exactly 1 u.

Why D is wrong: D is wrong because nitrogen-14 has never been used as the reference standard for atomic mass units.

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MCQ 2Easy RecallPractice

The atomic mass of an element represents:

Show answer and why every option is right or wrong

Answer: C. Atomic mass is the weighted average of isotopic masses based on natural abundance (NCERT Class 11 Chemistry Chapter 1, page 17).

Why A is wrong: A is wrong because atomic mass accounts for ALL isotopes weighted by abundance, not just the most abundant one. Chlorine's atomic mass is 35.5 u, not 35.

Why B is wrong: B is wrong because that describes mass number (A), an integer count, not the weighted average atomic mass.

Why D is wrong: D is wrong because molar mass in g/mol is numerically equal to atomic mass in u, but atomic mass is not defined per mole or in kilograms.

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MCQ 3Easy RecallPractice

The molecular mass of H₂O is closest to:

Show answer and why every option is right or wrong

Answer: D. H₂O: 2(1.008) + 16.00 = 18.02 u, closest to 18 u (NCERT Class 11 Chemistry Chapter 1, page 17).

Why A is wrong: A is wrong because 16 u is the atomic mass of oxygen alone; you must add the two hydrogen atoms (2 × 1.008 u).

Why B is wrong: B is wrong because 17 u accounts for only one hydrogen atom instead of two.

Why C is wrong: C is wrong because there is no standard molecular formula of water that yields 20 u; this may result from incorrectly adding mass numbers (2 + 2 + 16 = 20 by treating H as mass 2, which is deuterium).

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MCQ 4Direct ApplicationPractice

Calculate the molecular mass of Ca(OH)₂. (Atomic masses: Ca = 40.08 u, O = 16.00 u, H = 1.008 u)

Show answer and why every option is right or wrong

Answer: A. Ca(OH)₂: 40.08 + 2(16.00 + 1.008) = 40.08 + 2(17.008) = 40.08 + 34.016 = 74.096 ≈ 74.10 u. Expand the bracket before summing.

Why B is wrong: B is wrong because 57.09 u results from counting only one OH group instead of two: 40.08 + 16.00 + 1.008 ≈ 57.09.

Why C is wrong: C is wrong because 58.10 u likely results from treating the subscript 2 as applying only to H and not to the entire OH group: 40.08 + 16.00 + 2(1.008) ≈ 58.10.

Why D is wrong: D is wrong because 76.11 u counts two hydrogens in each OH group: 40.08 + 2(16.00 + 2 × 1.008) = 76.11.

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MCQ 5Direct ApplicationPractice

The atomic mass of boron is 10.81 u. Boron has two stable isotopes: ¹⁰B (mass 10.013 u) and ¹¹B (mass 11.009 u). What is the approximate percentage abundance of ¹⁰B?

Show answer and why every option is right or wrong

Answer: A. Let x = fraction of ¹⁰B. Then 10.013x + 11.009(1 − x) = 10.81. Solving: 10.013x + 11.009 − 11.009x = 10.81 → −0.996x = −0.199 → x ≈ 0.20 = 20%.

Why B is wrong: B is wrong because 80% is the abundance of ¹¹B, not ¹⁰B. The heavier isotope is more abundant since 10.81 is closer to 11 than to 10.

Why C is wrong: C is wrong because if both isotopes were equally abundant, the average would be (10.013 + 11.009)/2 = 10.511 u, not 10.81 u.

Why D is wrong: D is wrong because plugging x = 0.35 gives 10.013(0.35) + 11.009(0.65) = 3.505 + 7.156 = 10.66 u, which does not match 10.81 u.

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MCQ 6Direct ApplicationPractice

Which of the following has the highest molecular mass?

Show answer and why every option is right or wrong

Answer: B. Glucose: 6(12) + 12(1) + 6(16) = 180 u. Sucrose: 12(12) + 22(1) + 11(16) = 342 u. Ca₃(PO₄)₂: 3(40) + 2(31) + 8(16) = 310 u. H₂SO₄: 2(1) + 32 + 4(16) = 98 u. Sucrose at 342 u is highest.

Why A is wrong: A is wrong because glucose has molecular mass 180 u, which is less than sucrose (342 u) and Ca₃(PO₄)₂ (310 u).

Why C is wrong: C is wrong because Ca₃(PO₄)₂ has formula mass 310 u, which is less than sucrose (342 u). Note: this is technically a formula mass since Ca₃(PO₄)₂ is ionic.

Why D is wrong: D is wrong because H₂SO₄ has molecular mass 98 u, the lowest of the four options.

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MCQ 7Concept TrapPractice

A student is asked to find the "molecular mass of NaCl." The most accurate response is:

Show answer and why every option is right or wrong

Answer: D. NaCl is an ionic compound forming a crystal lattice, not discrete molecules. The correct term is formula mass (sum of atomic masses per formula unit = 22.99 + 35.45 = 58.44 u). NCERT Class 11 Chemistry Chapter 1 distinguishes molecular mass from formula mass for this reason.

Why A is wrong: A is wrong because calling it 'molecular mass' is technically incorrect — NaCl does not form molecules. The numerical value is right, but the terminology matters in conceptual questions.

Why B is wrong: B is wrong because formula mass uses atomic masses of neutral atoms (Na and Cl), not ionic masses. The electron mass difference is negligible but the conceptual framing of 'ion masses' is not the standard definition.

Why C is wrong: C is wrong because ionic compounds do have a definable formula mass; the concept of mass per formula unit is well-defined even without discrete molecules.

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MCQ 8CalculationPractice

Copper has two stable isotopes: ⁶³Cu (mass 62.93 u, abundance 69.2%) and ⁶⁵Cu (mass 64.93 u, abundance 30.8%). Calculate the molecular mass of CuSO₄. (Atomic masses: S = 32.06 u, O = 16.00 u)

Show answer and why every option is right or wrong

Answer: C. Step 1 — Atomic mass of Cu: 62.93(0.692) + 64.93(0.308) = 43.55 + 20.00 = 63.55 u. Step 2 — Molecular mass of CuSO₄: 63.55 + 32.06 + 4(16.00) = 63.55 + 32.06 + 64.00 = 159.61 u.

Why A is wrong: A is wrong because 161.06 u results from rounding Cu's atomic mass to 65 u (the mass number of ⁶⁵Cu) instead of computing the weighted average: 65 + 32.06 + 64.00 = 161.06.

Why B is wrong: B is wrong because 160.99 u results from using the mass of ⁶⁵Cu (64.93 u) directly as the atomic mass of copper without computing the weighted average: 64.93 + 32.06 + 64.00 = 160.99.

Why D is wrong: D is wrong because 158.99 u results from using the mass of ⁶³Cu (62.93 u) directly as copper's atomic mass, ignoring the ⁶⁵Cu contribution: 62.93 + 32.06 + 64.00 = 158.99.

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Atomic Molecular Masses: quick recall before you leave

How do you solve a Atomic Molecular Masses question? A worked example

  1. 1

    Given

    • Mass of ³⁵Cl = 34.97 u• Mass of ³⁷Cl = 36.97 u• Atomic mass of Cl = 35.45 u

  2. 2

    Required

    (a) % abundance of each isotope. (b) Molecular mass of Cl₂.

  3. 3

    Concept

    Atomic mass is the weighted average of isotopic masses. Once atomic mass is known, molecular mass of a diatomic molecule is simply 2 × atomic mass.

  4. 4

    Formula

    Weighted average: A = m₁x + m₂(1 − x), where x = fractional abundance of ³⁵Cl.
    Molecular mass: M(Cl₂) = 2 × A(Cl).

  5. 5

    Substitution

    34.97x + 36.97(1 − x) = 35.45

  6. 6

    Calculation

    34.97x + 36.97 − 36.97x = 35.45
    −2.00x = 35.45 − 36.97
    −2.00x = −1.52
    x = 0.76

    So ³⁵Cl abundance = 76%, ³⁷Cl abundance = 24%.

    Molecular mass of Cl₂ = 2 × 35.45 = 70.90 u.

    Note on exact values: The factor 2 in Cl₂ is an exact counting integer (number of atoms in the molecule) and does not limit significant figures.

  7. 7

    Final answer

    (a) ³⁵Cl: 76.0%, ³⁷Cl: 24.0%
    (b) M(Cl₂) = 70.90 u

  8. 8

    Common trap

    Rounding chlorine's atomic mass to 35 u (dropping the 0.45 contribution from ³⁷Cl) gives a molecular mass of 70 u — a full unit off. The decimal part of atomic mass directly encodes isotopic composition and must be preserved.

  9. 9

    Similar NEET-style question

    Silver has two stable isotopes: ¹⁰⁷Ag (mass 106.90 u, abundance 51.8%) and ¹⁰⁹Ag (mass 108.90 u, abundance 48.2%). Calculate the atomic mass of silver and the formula mass of AgNO₃. (N = 14.01 u, O = 16.00 u)

    ---

What to remember before solving Atomic Molecular Masses questions

Definition

Atomic mass unit

However, the present system of atomic masses is based on carbon-12 as the standard and has been agreed upon in 1961. Here, Carbon-12 is one of the isotopes of carbon and can be represented as 12C. In this system, 12C is assigned a mass of exactly 12 atomic mass unit (amu) and masses of all other atoms are given relative to this standard. One atomic mass unit is defined as a mass exactly equal to one-twelfth of the mass of one carbon – 12 atom. And 1 amu = 1.66056×10^–24 g … At present, ‘amu’ has been replaced by ‘u’, which is known as unified mass.

-- NCERT Class 11 Chemistry, Ch. 1, p. 16

More in Some Basic Concepts of Chemistry: 7 exam traps and mistakes · 3 formulas · 1 question pattern from its other lessons.

Atomic Molecular Masses questions from past NEET papers

No question in our NEET 2020–2025 set targets this topic directly.

All 12 past-paper questions from Some Basic Concepts of Chemistry →

Sources

NCERT refs: Class 11 Chemistry Chapter 1, p.17

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

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