Definition
Atomic mass unit
1 u = 1/12 of mass of one carbon-12 atom = 1.66 × 10⁻²⁷ kg. Used for atomic and molecular masses.
-- NCERT Class 11 Chemistry, Ch. 1, p. 8Atomic Molecular Masses
8 MCQs1 revision card9-step worked example
Source: NCERT Some Basic Concepts of ChemistryPYQ coverage: NEET 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
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 8). 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.
Practice 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:
MCQ 2Easy RecallPractice
The atomic mass of an element represents:
MCQ 3Easy RecallPractice
The molecular mass of H₂O is closest to:
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)
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?
MCQ 6Direct ApplicationPractice
Which of the following has the highest molecular mass?
MCQ 7Concept TrapPractice
A student is asked to find the "molecular mass of NaCl." The most accurate response is:
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)
Quick recall before you leave
Worked Example
- 1
Given
- Mass of ³⁵Cl = 34.97 u - Mass of ³⁷Cl = 36.97 u - Atomic mass of Cl = 35.45 u
- 2
Required
(a) % abundance of each isotope. (b) Molecular mass of Cl₂.
- 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
Formula
Weighted average: A = m₁x + m₂(1 − x), where x = fractional abundance of ³⁵Cl. Molecular mass: M(Cl₂) = 2 × A(Cl).
- 5
Substitution
34.97x + 36.97(1 − x) = 35.45
- 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
Final answer
(a) ³⁵Cl: 76.0%, ³⁷Cl: 24.0% (b) M(Cl₂) = 70.90 u
- 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
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) ---
Before solving, remember these
Formulas
Molality
Molal concentration: moles of solute per kg of solvent. Temperature-independent.
| Symbol | Quantity | SI Unit |
|---|---|---|
| m | molality | mol/kg |
| n | moles solute | mol |
Valid when
- Mass of SOLVENT (not solution)
Molarity
Molar concentration: moles of solute per litre of solution.
| Symbol | Quantity | SI Unit |
|---|---|---|
| M | molarity | mol/L |
| n | moles solute | mol |
| V | solution volume | L |
Valid when
- Volume of SOLUTION not solvent
- Temperature dependent (volume changes with T)
Number of moles
Three equivalent ways to compute moles: from mass and molar mass, from number of particles, from gas volume at STP.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | moles | mol |
| m | mass | g |
| M | molar mass | g/mol |
| N | particle count | - |
| NA | Avogadro 6.022e23 | 1/mol |
| V | gas volume | L |
Valid when
- Use g/(g/mol) for mass route
- STP for gas volume route
- Pure substance
Exam Traps & Common Mistakes
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Overthinking
Student picks reagent with smaller absolute moles as limiting, ignoring stoichiometric coefficients.
When it triggers
Stoichiometry problem with two reactants.
How to avoid
Compute (moles available / coefficient) for each. Smaller value = limiting reagent. Compare ratios, not raw moles.
Category: Overthinking
Student computes from total mole amounts without checking stoichiometric ratios. Limiting reagent is the one that runs out first; rest is excess.
When it triggers
Stoichiometry problem with two reactants given.
How to avoid
Compare moles available to stoichiometric requirement. Compute moles_actual / coefficient for each; smaller value = limiting reagent.
Category: Unit Conversion
Student uses 22.4 L/mol at non-STP conditions or assumes ideal-gas-only molar volume.
When it triggers
Question gives gas at non-STP T or P.
How to avoid
22.4 L/mol applies ONLY at STP (273.15 K, 100 kPa). For other conditions, use PV=nRT.
Category: Overthinking
Student forgets to multiply by purity% before computing moles from impure sample mass.
When it triggers
Question states sample is X% pure (e.g. limestone, ore).
How to avoid
Effective mass = sample mass × (purity/100). Then compute moles using effective mass / molar mass.
Root cause: concept gap
Correction
Compute (moles_actual / coefficient) for each reactant. Smaller value = limiting.
Root cause: concept gap
Correction
22.4 L/mol applies ONLY at STP. Use PV=nRT for other conditions.
Root cause: concept gap
Correction
Effective mass = stated mass × (purity/100). Then convert to moles using molar mass.
Past Year Questions
4 questions from NEET 2023, 2024, 2025. Answers verified against NTA official keys.
NEET 2025
Dalton’s Atomic theory could not explain which of the following?
1Law of gaseous volume
2Law of conservation of mass
3Law of constant proportion
4Law of multiple proportion
NTA Answer: Option 1(final)
NEET 2024Revised key
1750 mg
2250 mg
3Zero mg
4200 mg
NTA Answer: Option 2(revised_final)
NEET 2024Revised key
1A BC 2 2
2ABC 3
3AB C 2 2
4ABC 4
NTA Answer: Option 2(revised_final)
NEET 2023
11.76 g
22.64 g
31.32 g
41.12 g
NTA Answer: Option 1(final)
How NEET usually asks this
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Mole concept and stoichiometry calculations: mole-mass conversion, limiting reagent, percentage purity, % composition.
Direct ApplicationMedium
Common distractors
swapped classes
Tempts surface-level recall.
Sources
NCERT refs: Class 11 Chemistry Chapter 1, p.8
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