JEE Mains Chemistry · Teaching notes

Some Basic Concepts of Chemistry — JEE Mains Chemistry

Some Basic Concepts of Chemistry has 205 past-year questions from 2021 to 2026, and 111 of them ask for a number rather than a choice of option. Almost every one is arithmetic on the mole: turn what the stem gives into moles, apply a ratio from a formula or a balanced equation, and turn the answer back into grams, litres or a concentration. The bank also files gas laws, oxidation numbers and titrations under this chapter, so they are taught here too. Order matters, because concentration needs the mole and titrations need both concentration and the electron counts from redox balancing. The ideas are not hard; units, molar volumes and n-factors decide the marks.

Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.

All 205 past questions JEE Mains Chemistry strategy JEE Mains home

Subtopic notes

Formula & revision sheet

19 formulas · 4 reference tables · 31 gotchas across all subtopics — the exam-eve cheat-sheet

Measurement and the Mole

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

Percentage Composition and Empirical Formula

Formulas (3)

Mass percentage of an element · Mass percentage $\%X=\frac{n_X\,A_X}{M}\times100$
Empirical and molecular formula · Molecular formula $\text{MF}=\left(\frac{M}{\text{EF mass}}\right)\times\text{EF}$
Formula from combustion volumes · Combustion of a hydrocarbon $\mathrm{C}_x\mathrm{H}_y+\left(x+\tfrac{y}{4}\right)\mathrm{O_2}\rightarrow x\,\mathrm{CO_2}+\tfrac{y}{2}\,\mathrm{H_2O}$

Watch out for (4)

Carbon is 12/44 of CO₂
→ Mass percentage of an element
Rounding 1.5 away
→ Empirical and molecular formula
Empirical mass offered as the molar mass
→ Empirical and molecular formula
The water is gone after cooling
→ Formula from combustion volumes

Reaction Stoichiometry and Limiting Reagent

Formulas (2)

Mole ratios, purity and yield · Mass to mass $m_B=\frac{m_A}{M_A}\times\frac{b}{a}\times M_B$
Limiting reagent and the excess left · Limiting reagent test $\text{limiting reagent}=\text{the smallest }\frac{n_i}{\nu_i}$

Watch out for (4)

Silver carbonate leaves silver
→ Mole ratios, purity and yield
Using the impure mass
→ Mole ratios, purity and yield
Fewest moles is not the test
→ Limiting reagent and the excess left
Product from the excess reagent
→ Limiting reagent and the excess left

Gas Laws and Gas Volumes

Formulas (3)

The ideal gas equation and its special cases · Ideal gas equation $PV=nRT=\frac{m}{M}RT\quad\Rightarrow\quad M=\frac{dRT}{P}$
Gas volumes in reactions · Volume from moles $V_{\text{gas}}=n\,V_m,\qquad\frac{V_A}{V_B}=\frac{\nu_A}{\nu_B}$
Partial pressures and gas mixtures · Dalton's law $p_i=x_iP=\frac{n_i}{n_{\text{total}}}\,P$

Watch out for (5)

Celsius in the gas equation
→ The ideal gas equation and its special cases
Match R to the units
→ The ideal gas equation and its special cases
Only gases count
→ Gas volumes in reactions
Mass fraction is not mole fraction
→ Partial pressures and gas mixtures
Moist gas: only the dry part changes
→ Partial pressures and gas mixtures

Molarity, Dilution and Mixing

Formulas (2)

Molarity from mass and volume · Molarity $M=\frac{n_{\text{solute}}}{V_{\text{solution}}(\mathrm{L})}$
Dilution and mixing · Mixing rule $M_{\text{mix}}=\frac{M_1V_1+M_2V_2}{V_1+V_2}$

Reference tables (1)

Which concentration terms depend on temperature7 rows

Term	Defined with	Changes with temperature?
Molarity	Volume of solution (L)	Yes
Normality	Volume of solution (L)	Yes
Molality	Mass of solvent (kg)	No
Mole fraction	Moles only	No
Mass percent	Masses only	No
ppm (by mass)	Masses only	No
Mole	An amount of substance	Not a concentration term at all In a list of 'units of concentration', the mole is the one to leave out.

Water is densest at 4 °C, so the molarity of an aqueous solution peaks near 4 °C.

Watch out for (3)

Forgetting the water of crystallisation
→ Molarity from mass and volume
Water added is not the final volume
→ Dilution and mixing
Heating does not always lower molarity
→ Which concentration terms depend on temperature

Molality, Mole Fraction and ppm

Formulas (3)

Converting molarity, molality and mass percent · Molarity to molality $m=\frac{1000\,M}{1000\,d-M\,M_B}$
Mole fraction and mass percent of mixtures · Mole fraction from molality $x_{\text{solute}}=\frac{m}{m+\frac{1000}{M_{\text{solvent}}}}$
Parts per million · Parts per million $\text{ppm}=\frac{m_{\text{solute}}}{m_{\text{solution}}}\times10^{6}$

Watch out for (3)

Dividing by the solution's mass
→ Converting molarity, molality and mass percent
Whose mole fraction?
→ Mole fraction and mass percent of mixtures
The element, not the compound
→ Parts per million

Oxidation Number and Redox Reactions

Formulas (2)

Assigning oxidation numbers · Charge balance $\sum(\text{oxidation numbers})=\text{charge on the species}$
Balancing redox equations by half-reactions · Electron balance $\text{electrons lost in oxidation}=\text{electrons gained in reduction}$

Reference tables (1)

Types of redox reaction7 rows

Reaction	Type	Why
$\mathrm{2Mg+O_2\rightarrow2MgO}$	Combination	Two reactants, one product
$\mathrm{2Pb(NO_3)_2\rightarrow2PbO+4NO_2+O_2}$	Decomposition	N goes from $+5$ to $+4$ ; O goes from $-2$ to 0
$\mathrm{V_2O_5+5Ca\rightarrow2V+5CaO}$	Metal displacement	Ca pushes V out of its oxide
$\mathrm{2Na+2H_2O\rightarrow2NaOH+H_2}$	Hydrogen displacement	Na pushes H out of water
$\mathrm{2H_2O_2\rightarrow2H_2O+O_2}$	Disproportionation	O at $-1$ goes to $-2$ and to 0
$\mathrm{Cl_2+2OH^-\rightarrow Cl^-+ClO^-+H_2O}$	Disproportionation	Cl at 0 goes to $-1$ and to $+1$
$\mathrm{2MnO_4^-+3Mn^{2+}+2H_2O\rightarrow5MnO_2+4H^+}$	Comproportionation	$+7$ and $+2$ meet at $+4$ ; not a disproportionation

Disproportionation needs one element in one intermediate oxidation state going both up and down.

Watch out for (5)

A fraction is an average
→ Assigning oxidation numbers
Burning methane is 'combination' here
→ Types of redox reaction
Two manganese species are not enough
→ Types of redox reaction
The medium sets the product
→ Balancing redox equations by half-reactions
Check the charge, not just the atoms
→ Balancing redox equations by half-reactions

Equivalents and Titrations

Formulas (2)

Acid–base titration and the n-factor · End point $M_1n_1V_1=M_2n_2V_2$
Redox titrations: permanganate, dichromate and iodometry · Redox end point $M_1n_1V_1=M_2n_2V_2,\qquad n=\text{electrons per formula unit}$

Reference tables (1)

Primary standards9 rows

Substance	Primary standard?	Reason
Oxalic acid dihydrate, $\mathrm{H_2C_2O_4\cdot2H_2O}$	Yes	Stable crystals of fixed composition
Potassium hydrogen phthalate (KHP)	Yes	High molar mass, not hygroscopic; standardises NaOH with phenolphthalein
Mohr's salt, $\mathrm{(NH_4)_2Fe(SO_4)_2\cdot6H_2O}$	Yes	Its $\mathrm{Fe^{2+}}$ resists air oxidation, unlike ferrous sulphate
$\mathrm{K_2Cr_2O_7}$	Yes	Pure, stable and not hygroscopic
Borax, $\mathrm{Na_2B_4O_7\cdot10H_2O}$	Yes	Used to standardise acids
NaOH	No	Absorbs water and $\mathrm{CO_2}$ from air
$\mathrm{KMnO_4}$	No	Hard to get pure; slowly reduced by light and traces of organic matter
$\mathrm{Na_2Cr_2O_7}$	No	Hygroscopic, unlike the potassium salt
Ferrous sulphate hydrates	No	Air oxidises $\mathrm{Fe^{2+}}$ to $\mathrm{Fe^{3+}}$

The potassium dichromate is a standard; the sodium one is not.

Watch out for (3)

Forgetting n = 2
→ Acid–base titration and the n-factor
Count every oxidisable part
→ Redox titrations: permanganate, dichromate and iodometry
Hydrated salts can qualify
→ Primary standards

PYQ weightage by concept

23 concepts · 205 PYQs — where the marks actually sit, so you know what to drill first

Measurement and the Mole22 PYQs · 11%

Concept	PYQs	Share
Counting atoms, molecules and electrons	9	4%
Significant figures, SI units and Dalton's theory	7	3%
Mass, moles and molar volume	6	3%

Percentage Composition and Empirical Formula20 PYQs · 10%

Concept	PYQs	Share
Empirical and molecular formula	10	5%
Mass percentage of an element	6	3%
Formula from combustion volumes	4	2%

Reaction Stoichiometry and Limiting Reagent34 PYQs · 17%

Concept	PYQs	Share
Mole ratios, purity and yield	18	9%
Limiting reagent and the excess left	16	8%

Gas Laws and Gas Volumes30 PYQs · 15%

Concept	PYQs	Share
The ideal gas equation and its special cases	16	8%
Gas volumes in reactions	7	3%
Partial pressures and gas mixtures	7	3%

Molarity, Dilution and Mixing18 PYQs · 9%

Concept	PYQs	Share
Molarity from mass and volume	10	5%
Dilution and mixing	4	2%
Which concentration terms depend on temperature	4	2%

Molality, Mole Fraction and ppm26 PYQs · 13%

Concept	PYQs	Share
Converting molarity, molality and mass percent	13	6%
Mole fraction and mass percent of mixtures	9	4%
Parts per million	4	2%

Oxidation Number and Redox Reactions24 PYQs · 12%

Concept	PYQs	Share
Assigning oxidation numbers	9	4%
Types of redox reaction	8	4%
Balancing redox equations by half-reactions	7	3%

Equivalents and Titrations31 PYQs · 15%

Concept	PYQs	Share
Redox titrations: permanganate, dichromate and iodometry	13	6%
Acid–base titration and the n-factor	12	6%
Primary standards	6	3%

Test yourself on Some Basic Concepts of Chemistry

20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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Measurement and the Mole

Percentage Composition and Empirical Formula

Reaction Stoichiometry and Limiting Reagent

Gas Laws and Gas Volumes

Molarity, Dilution and Mixing

Molality, Mole Fraction and ppm

Oxidation Number and Redox Reactions

Equivalents and Titrations

Formula & revision sheet

PYQ weightage by concept

Test yourself on Some Basic Concepts of Chemistry

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