JEE Mains Chemistry · Some Basic Concepts of Chemistry

Measurement and the Mole

Significant figures, SI units and Dalton's postulates, then the mole: n = m/M = N/Nₐ = V/Vₘ, used to count atoms, molecules and electrons.

All Some Basic Concepts of Chemistry notes JEE Mains Chemistry strategy

Why this matters

Twenty-two PYQs, sixteen of them multiple choice and four from 2026. Seven test measurement: significant figures, SI units, mass against weight and Dalton's postulates. Nine count the atoms, molecules or electrons in a given amount, and six convert between mass, moles and gas volume. Every later page in the chapter starts with n = m/M.

Concept 1 of 3: Significant figures, SI units and Dalton's theory

These are rule questions, not calculations. Count significant figures with the four zero rules, and round a product or quotient to the fewest figures in its data. The rest is a short list of definitions that the paper quotes back with one word changed.

Definition

Non-zero digits always count. Zeros between non-zero digits always count.
Leading zeros never count: $0.0047$ has 2.
Trailing zeros count only when there is a decimal point: $2.500$ has 4.
A power of ten never changes the count: $3.20\times10^{5}$ has 3.
For $\times$ and $\div$ , keep the fewest significant figures in the data. For $+$ and $-$ , keep the fewest decimal places.
Mass is the amount of matter; weight is the force of gravity on it. The kelvin scale has no negative values. Precision is how close repeated readings are; accuracy is how close they are to the true value.
Dalton: atoms are indivisible; atoms of one element are identical in mass and properties; atoms combine in a fixed, simple whole-number ratio; a reaction only rearranges atoms.

Number or statement	Rule	Result
$0.0047$	Leading zeros never count	2 significant figures
$3.050$	Captive zeros and trailing zeros after a decimal point count	4 significant figures
$6.20\times10^{-4}$	The power of ten is ignored	3 significant figures
$\frac{4.52\times1.3}{2.001}$	Keep the fewest significant figures (1.3 has 2)	$2.9$
$12.11+0.3+1.024$	Keep the fewest decimal places (0.3 has 1)	$13.4$
SI base units	Seven: m, kg, s, A, K, mol, cd	The candela uses $540\times10^{12}$ Hz and $\frac{1}{683}$ W sr $^{-1}$
Dalton's combining rule	Atoms combine in a fixed, simple whole-number ratio	'Any ratio' is false 'Atoms are divisible' and 'atoms of one element differ in mass' are the other planted false postulates.
Mass and weight	Mass is matter; weight is a force	A statement that swaps them is false

A power of ten never adds or removes a significant figure.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 8 Apr 2023 · Q121Moderate

Example 1 · Some Basic Concepts of Chemistry · Measurement and the Mole

Which of the following have same number of significant figures? (A) 0.00253 (B) 1.0003 (C) 15.0 (D) 163 Choose the correct answer from the options given below

A power of ten hides no digits

50.0\times10^{3}

has 3 significant figures, not 5. Count only the digits in front of the power of ten.

Precision is not accuracy

Readings of 2.31, 2.32 and 2.31 are precise even if the true value is 2.50. Precision compares readings with each other; accuracy compares them with the true value.

Drill 6 more on significant figures, si units and dalton's theory

Concept 2 of 3: Counting atoms, molecules and electrons

Find the moles first, then count. The number of molecules is

n\,N_A

. Multiply by the atoms in one formula unit to count atoms, or by the electrons in one molecule to count electrons.

Definition

$N=n\,N_A$ , with $N_A=6.022\times10^{23}\ \mathrm{mol^{-1}}$ .
Atoms $=n\times$ atoms per formula unit: $\mathrm{H_2O}$ 3, $\mathrm{C_6H_{12}}$ 18, $\mathrm{C_{12}H_{22}O_{11}}$ 45.
Atoms of one element $=n\times$ its subscript: $\mathrm{C_7H_5N_3O_6}$ has 3 N atoms per molecule.
Electrons $=n\times$ electrons per molecule: $\mathrm{CH_4}$ has $6+4=10$ , $\mathrm{N_2}$ has 14.
Equal masses of different elements: the smallest atomic mass gives the most atoms.
From a mass percent: mass of the element $=$ percent $\times$ sample mass, then divide by its atomic mass.

Number of particles

N=\frac{m}{M}\,N_A\times(\text{particles per formula unit})

Worked example

How many oxygen atoms, and how many electrons, are there in

8.8

g of

\mathrm{CO_2}

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 8 Apr 2026 Shift 2 · Q26Moderate

Example 2 · Some Basic Concepts of Chemistry · Measurement and the Mole

Match List-I with List-II.

	List-I Mass of substance		List-II Number of atoms
(A)	$1.8mg$ water	(I)	$2 \times 10^{- 4} \times N_{A}$
(B)	$9.8mg$ sulphuric acid	(II)	$1.5 \times 10^{- 4} \times N_{A}$
(C)	1.8 mg carbon	(III)	$3 \times 10^{- 4} \times N_{A}$
(D)	5.85 mg salt $(NaCl)$	(IV)	$7 \times 10^{- 4} \times N_{A}$

Choose the correct answer from the options given below:

Molecules are not atoms

0.1

mol of

\mathrm{O_2}

holds

0.1N_A

molecules but

0.2N_A

atoms. Options usually offer both; read which one the question asks for.

Drill 8 more on counting atoms, molecules and electrons

Concept 3 of 3: Mass, moles and molar volume

The mole links three things you can measure: a mass, a number of particles and a gas volume. For a gas, use the molar volume that matches the conditions in the stem.

Definition

$n=\frac{m}{M}=\frac{N}{N_A}=\frac{V}{V_m}$ .
$V_m=22.4$ L at 273 K and 1 atm; $V_m=22.7$ L at 273.15 K and 1 bar.
If the stem names no molar volume, try both: the intended one gives round numbers.
Equimolar mixture: call the common amount $n$ and add the masses.
Two samples of equal mass: write each mass as moles $\times$ molar mass and set them equal.

Mole relations

n=\frac{m}{M}=\frac{N}{N_A}=\frac{V}{V_m}

Worked example

Find the mass and the number of molecules in

5.6

L of

\mathrm{N_2}

at 273 K and 1 atm.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 4 Apr 2026 Shift 1 · Q26Moderate

Example 3 · Some Basic Concepts of Chemistry · Measurement and the Mole

Number of moles and number of molecules in 1.4187 L of

SO_{2}

at STP respectively are

22.4 or 22.7

The two molar volumes differ by about 1.3%, enough to change a four-figure answer. 273 K and 1 atm gives 22.4 L; 273.15 K and 1 bar gives 22.7 L.

Drill 5 more on mass, moles and molar volume

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Counting atoms, molecules and electrons
Number of particles
$N=\frac{m}{M}\,N_A\times(\text{particles per formula unit})$
Mass, moles and molar volume
Mole relations
$n=\frac{m}{M}=\frac{N}{N_A}=\frac{V}{V_m}$

Reference tables (1)

Significant figures, SI units and Dalton's theory8 rows

Number or statement	Rule	Result
$0.0047$	Leading zeros never count	2 significant figures
$3.050$	Captive zeros and trailing zeros after a decimal point count	4 significant figures
$6.20\times10^{-4}$	The power of ten is ignored	3 significant figures
$\frac{4.52\times1.3}{2.001}$	Keep the fewest significant figures (1.3 has 2)	$2.9$
$12.11+0.3+1.024$	Keep the fewest decimal places (0.3 has 1)	$13.4$
SI base units	Seven: m, kg, s, A, K, mol, cd	The candela uses $540\times10^{12}$ Hz and $\frac{1}{683}$ W sr $^{-1}$
Dalton's combining rule	Atoms combine in a fixed, simple whole-number ratio	'Any ratio' is false 'Atoms are divisible' and 'atoms of one element differ in mass' are the other planted false postulates.
Mass and weight	Mass is matter; weight is a force	A statement that swaps them is false

A power of ten never adds or removes a significant figure.

Watch out for (4)

A power of ten hides no digits
→ Significant figures, SI units and Dalton's theory
Precision is not accuracy
→ Significant figures, SI units and Dalton's theory
Molecules are not atoms
→ Counting atoms, molecules and electrons
22.4 or 22.7
→ Mass, moles and molar volume

Test yourself on Some Basic Concepts of Chemistry

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