Mole Concept

8 MCQs3 revision cards9-step worked example
Source: NCERT Some Basic Concepts of ChemistryPYQ coverage: NEET 2020, 2024, 2025, 2026Official key: NTA-verifiedLast updated: 26 Sep 2026

Mole Concept, explained for NEET

The trap that costs marks: You apply 22.4 L/mol to find moles of a gas — but the question quietly states the temperature is 37 °C, not 0 °C. The 22.4 figure is invalid. You lose 5 marks (4 + 1 negative).

The mole defined. One mole is the amount of substance containing exactly 6.022 × 10²³ particles (atoms, molecules, ions, or formula units). The molar mass of a substance in g/mol is numerically equal to its atomic/molecular mass in amu (NCERT Class 11 Chemistry Chapter 1, page 18).

Three routes to moles. The formula n = m/M = N/Nₐ = V/22.4 gives three equivalent paths:

  1. Mass route: n = mass (g) ÷ molar mass (g/mol)
  2. Particle route: n = number of particles ÷ 6.022 × 10²³
  3. Gas-volume route: n = volume (L) ÷ 22.4 — only at STP (273.15 K, 1 bar)

(NCERT Class 11 Chemistry Chapter 1, page 12)

The STP trap in detail. 22.4 L/mol is the molar volume of an ideal gas at STP (273.15 K, 100 kPa). If the question specifies any other temperature or pressure, you must use PV = nRT instead. NEET setters exploit this by burying a non-standard temperature in the problem data while making 22.4 L the tempting shortcut.

Rounding off a calculated answer. Mole calculations end in a number that must be cut to the right number of significant figures, and NCERT Class 11 Chemistry, Chapter 1, page 13 gives three rules for dropping the rightmost digit. (1) If the digit removed is more than 5, the preceding digit goes up by one: 1.386 becomes 1.39. (2) If it is less than 5, the preceding digit is unchanged: 4.334 becomes 4.33. (3) If it is exactly 5, the preceding digit stays as it is when it is even and goes up by one when it is odd: 6.35 becomes 6.4, but 6.25 becomes 6.2. Rule 3 is the one that catches students who always "round 5 up". Apply it to 2.0465 and 3.0175 cut to three decimal places: 6 is even, so 2.0465 becomes 2.046; 7 is odd, so 3.0175 becomes 3.018. The same page shows the limit in a product: 2.5 × 1.25 = 3.125 is reported as 3.1, because 2.5 has only two significant figures. The counting rules for significant figures themselves are taught in the Physics lesson on significant figures.

Watch-out: When converting mass to moles, confirm whether the question asks for moles of atoms or moles of molecules. For O₂ (M = 32 g/mol), 32 g = 1 mol of O₂ molecules but 2 mol of O atoms.

Can you answer these Mole Concept MCQs?

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

MCQ 1Direct ApplicationPractice

What is the number of molecules in 11.2 L of CO₂ at STP?

Show answer and why every option is right or wrong

Answer: D. At STP, n = 11.2/22.4 = 0.5 mol. Number of molecules = 0.5 × 6.022 × 10²³ = 3.011 × 10²³ (NCERT Class 11 Chemistry Chapter 1, page 12).

Why A is wrong: A assumes 11.2 L = 1 mol (uses 22.4 without dividing correctly, or confuses with 22.4 L = 1 mol directly). Trap: failing to compute n = V/22.4 properly.

Why B is wrong: B doubles the Avogadro number, treating 11.2 L as 2 mol — arithmetic error in division.

Why C is wrong: C halves the correct answer again (divides by 4 instead of 2), likely confusing atom count per molecule with total molecule count.

Report an error · Every fix is public: corrections log

MCQ 2Direct ApplicationPractice

The molar mass of calcium carbonate (CaCO₃) is 100 g/mol. How many moles are in 25 g of CaCO₃?

Show answer and why every option is right or wrong

Answer: D. n = m/M = 25/100 = 0.25 mol. Direct mass-to-mole conversion (NCERT Class 11 Chemistry Chapter 1, page 12).

Why A is wrong: A adds an extra factor of 10 in the denominator (divides by 1000 instead of 100).

Why B is wrong: B inverts the formula (computes M/m = 100/25 = 4). Trap: formula inversion.

Why C is wrong: C misplaces the decimal (computes 25/10 instead of 25/100).

Report an error · Every fix is public: corrections log

MCQ 3Easy RecallPractice

One mole of any substance contains:

Show answer and why every option is right or wrong

Answer: C. By definition, one mole = 6.022 × 10²³ elementary entities (Avogadro's number). NCERT Class 11 Chemistry Chapter 1, page 18.

Why A is wrong: A is the reciprocal of Avogadro's number (1/Nₐ in grams per amu), not a particle count.

Why B is wrong: B is off by one power of ten — a common exponent-recall error.

Why D is wrong: D is off by three powers of ten.

Report an error · Every fix is public: corrections log

MCQ 4Direct ApplicationPractice

The number of atoms in 0.5 mol of nitrogen gas (N₂) is:

Show answer and why every option is right or wrong

Answer: A. 0.5 mol N₂ = 0.5 × 6.022 × 10²³ molecules. Each N₂ has 2 atoms, so atoms = 0.5 × 2 × 6.022 × 10²³ = 6.022 × 10²³ (NCERT Class 11 Chemistry Chapter 1, page 18).

Why B is wrong: B counts molecules (0.5 × Nₐ) but forgets the atomicity factor of 2. Trap: confusing molecules with atoms in diatomic species.

Why C is wrong: C halves again (gives 0.25 × Nₐ), applying atomicity in the wrong direction.

Why D is wrong: D doubles the correct answer (treats 0.5 mol as 1 mol and then doubles for atomicity).

Report an error · Every fix is public: corrections log

MCQ 5Easy RecallPractice

The SI unit of molar mass is:

Show answer and why every option is right or wrong

Answer: B. The SI unit of molar mass is kg/mol (mass in kg per amount in mol). g/mol is the commonly used unit in chemistry but is not the SI base-unit expression. NCERT Class 11 Chemistry Chapter 1, page 18 gives molar mass in grams per mole; kg mol⁻¹ as the SI expression goes beyond NCERT.

Why A is wrong: A (g/mol) is the conventional unit used in calculations but not the strict SI unit. Trap: confusing practical convention with SI definition.

Why C is wrong: C (amu) is the unit of atomic mass, not molar mass — these are numerically equal but dimensionally different.

Why D is wrong: D (g/L) is a unit of density, not molar mass.

Report an error · Every fix is public: corrections log

MCQ 6Direct ApplicationPractice

A gas occupies 44.8 L at STP. The number of moles of the gas is:

Show answer and why every option is right or wrong

Answer: A. At STP, n = V/22.4 = 44.8/22.4 = 2 mol (NCERT Class 11 Chemistry Chapter 1, page 12).

Why B is wrong: B results from dividing 22.4/22.4 = 1 (using the wrong numerator). Trap: misreading the given volume.

Why C is wrong: C inverts the division (22.4/44.8 = 0.5).

Why D is wrong: D doubles the correct answer (multiplies instead of divides, or uses 11.2 as molar volume).

Report an error · Every fix is public: corrections log

MCQ 7CalculationPractice

4.4 g of CO₂ (molar mass 44 g/mol) at STP occupies approximately:

Show answer and why every option is right or wrong

Answer: B. n = 4.4/44 = 0.1 mol. At STP, V = n × 22.4 = 0.1 × 22.4 = 2.24 L (NCERT Class 11 Chemistry Chapter 1, page 12). The STP condition is satisfied, so 22.4 L/mol applies.

Why A is wrong: A assumes 4.4 g = 1 mol (divides by 4.4 instead of 44). Trap: decimal-place error in molar mass.

Why C is wrong: C is off by a factor of 10 (uses 0.01 mol instead of 0.1 mol).

Why D is wrong: D assumes 4.4 g = 2 mol — arithmetic error.

Report an error · Every fix is public: corrections log

MCQ 8Concept TrapPractice

A sample of gas at 37 °C and 1 atm pressure occupies 22.4 L. A student claims this equals 1 mol of gas. What is wrong with this claim?

Show answer and why every option is right or wrong

Answer: C. The molar volume of 22.4 L/mol is valid only at STP (273.15 K, 100 kPa). At 37 °C (310.15 K), the gas occupies more than 22.4 L per mole, so 22.4 L at this temperature contains fewer than 1 mol. The correct approach is PV = nRT. (Trap: trap: mole volume stp)

Why A is wrong: A repeats the exact misconception the question tests — assuming 22.4 L/mol is universal regardless of temperature and pressure.

Why B is wrong: B fabricates a restriction on atomicity that does not exist; 22.4 L/mol applies to any ideal gas at STP, regardless of atomicity.

Why D is wrong: D gives a specific incorrect molar volume without proper derivation (actual value at 310.15 K, 1 atm ≈ 25.4 L, but the option's reasoning that 'the sample is less than 1 mol' is coincidentally directionally correct for the wrong computational reason — it doesn't identify the STP condition as the core issue).

Report an error · Every fix is public: corrections log

Free NEET study resources

Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.

Mole Concept: quick recall before you leave

How do you solve a Mole Concept question? A worked example

  1. 1

    Given

    • Mass of water (H₂O) = 3.6 g• Molar mass of H₂O = 18 g/mol (exact, by definition of molar mass from atomic masses: H = 1, O = 16)• Required: number of hydrogen atoms in the sample

  2. 2

    Required

    Find the total number of H atoms in 3.6 g of H₂O.

  3. 3

    Concept

    Each water molecule contains 2 hydrogen atoms. First find moles of H₂O, then molecules, then multiply by atomicity.

  4. 4

    Formula

    n = m/M; Number of molecules = n × Nₐ; Number of H atoms = molecules × 2

  5. 5

    Substitution

    n = 3.6 g ÷ 18 g/mol = 0.2 mol

  6. 6

    Calculation

    Molecules of H₂O = 0.2 × 6.022 × 10²³ = 1.2044 × 10²³

    H atoms = 1.2044 × 10²³ × 2 = 2.4088 × 10²³

    Note on exact constants: The factor 2 (atoms of H per molecule of H₂O) is a counting integer and does not limit significant figures. The molar mass 18 g/mol is treated as exact for this calculation.

  7. 7

    Final answer

    Number of H atoms = 2.41 × 10²³ (3 significant figures, matching the precision of 3.6 g which has 2 sig figs — answer reported to 3 sig figs as a conservative upper bound; 2 sig figs would give 2.4 × 10²³).

  8. 8

    Common trap

    A common error is reporting the number of O atoms or the number of molecules instead of H atoms. The atomicity multiplier (×2 for H in H₂O) is the step most frequently dropped. This connects to the mole-concept trap of confusing "moles of molecules" with "moles of atoms."

  9. 9

    Similar NEET-style question

    "Calculate the number of oxygen atoms in 0.5 mol of Al₂(SO₄)₃." (Answer requires recognizing 12 oxygen atoms per formula unit: 3 × 4 = 12 O atoms per molecule.)

What to remember before solving Mole Concept questions

Definition

Mole concept and Avogadro's number

1.8 Mole concept and Molar Masses … In SI system, mole (symbol, mol) was introduced as seventh base quantity for the amount of a substance. The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.02214076 ×10^23 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol–1 and is called the Avogadro number. The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles. It may be emphasised that the mole of a substance always contains the same number of entities, no matter what the substance may be. … This number of entities in 1 mol is so important that it is given a separate name and symbol. It is known as ‘Avogadro constant’, or Avogadro number denoted by NA in honour of Amedeo Avogadro. … We can, therefore, say that 1 mol of hydrogen atoms = 6.022 ×10^23 atoms 1 mol of water molecules = 6.022 ×10^23 water molecules 1 mol of sodium chloride = 6.022×10^23 formula units of sodium chloride Having defined the mole, it is easier to know the mass of one mole of a substance or the constituent entities. The mass of one mole of a substance in grams is called its molar mass. The molar mass in grams is numerically equal to atomic/molecular/formula mass in u.

-- NCERT Class 11 Chemistry, Ch. 1, p. 18
Formula

Mole, molecule, volume and mass relations read from a balanced equation

Let us consider the combustion of methane. A balanced equation for this reaction is as given below: CH4 (g) + 2O2 (g) → CO2 (g) + 2 H2O (g) … The coefficients 2 for O2 and H2O are called stoichiometric coefficients. Similarly the coefficient for CH4 and CO2 is one in each case. They represent the number of molecules (and moles as well) taking part in the reaction or formed in the reaction. Thus, according to the above chemical reaction, • One mole of CH4(g) reacts with two moles of O2(g) to give one mole of CO2(g) and two moles of H2O(g) • One molecule of CH4(g) reacts with 2 molecules of O2(g) to give one molecule of CO2(g) and 2 molecules of H2O(g) • 22.7 L of CH4(g) reacts with 45.4 L of O2 (g) to give 22.7 L of CO2 (g) and 45.4 L of H2O(g) • 16 g of CH4 (g) reacts with 2×32 g of O2 (g) to give 44 g of CO2 (g) and 2×18 g of H2O (g). From these relationships, the given data can be interconverted as follows: mass ⇌ moles ⇌ no. of molecules Mass / Volume = Density

-- NCERT Class 11 Chemistry, Ch. 1, p. 20
Key Fact

Rounding-off rule when removed digit is 5

When rounding off a number, if the rightmost digit to be removed is more than 5, the preceding digit is increased by one; if it is less than 5, the preceding digit is not changed; if it is exactly 5, the preceding digit is not changed if it is an even number but is increased by one if it is an odd number.

-- NCERT Class 11 Chemistry, Ch. 1, p. 13

Which Mole Concept formulas do you need for NEET?

Number of moles

Three equivalent ways to compute moles: from mass and molar mass, from number of particles, from gas volume at 273.15 K and 1 bar.

SymbolQuantitySI Unit
nmolesmol
mmassg
Mmolar massg/mol
Nparticle count-
NAAvogadro 6.022e231/mol
Vgas volumeL

Valid when

  • Use g/(g/mol) for mass route
  • 22.7 L mol^-1 only at 273.15 K and 1 bar (22.4 L mol^-1 if the question uses 1 atm); otherwise PV = nRT
  • Pure substance

Where do students lose marks on Mole Concept?

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Mole-volume STP confusion

Category: Unit Conversion

Student uses a fixed molar volume (22.7 L/mol or 22.4 L/mol) at conditions it does not apply to, or mixes the two conventions.

When it triggers

Question gives gas at non-STP T or P.

How to avoid

One mole of an ideal gas occupies 22.7 L at 273.15 K and 1 bar (100 kPa), the value NCERT's reprint uses; 22.4 L is the older figure for 273.15 K and 1 atm (101.325 kPa). Use whichever matches the conditions the question states; for any other T or P, use PV = nRT.

More in Some Basic Concepts of Chemistry: 5 exam traps and mistakes · 2 formulas from its other lessons.

Mole Concept questions from past NEET papers

4 questions from NEET 2020, 2024, 2025, 2026. Answers verified against NTA official keys.

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

How does NEET ask about Mole Concept?

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Sources

NCERT refs: Class 11 Chemistry Chapter 1, p.18 | Class 11 Chemistry Chapter 1, p.12 | Class 11 Chemistry Chapter 1, p.13

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.

Report an error · Every fix is public: corrections log

Test yourself on this topic with real past-paper questions:

Practice this topic →