JEE (Main) · Chemistry, Class 11
Some Basic Concepts of Chemistry: JEE Main PYQ, 2020–2025
Some Basic Concepts of Chemistry (Chemistry, Class 11) has been asked 133 times in the 111 JEE (Main) shift papers from 2020–2025: about 1.2 questions a paper, 5% of all Chemistry questions. It is the 9th most-asked of 21 Chemistry chapters. The 2025 shifts had 20 from it in all. Last checked 28 October 2026.
Questions per year, all shifts
Year by year.
| Year | Questions |
|---|---|
| 2020 | 17in 14 papers |
| 2021 | 31in 26 papers |
| 2022 | 38in 24 papers |
| 2023 | 13in 12 papers |
| 2024 | 14in 18 papers |
| 2025 | 20in 17 papers |
JEE (Main) is sat in many shifts a year, each with its own paper; the counts add up every shift. 2021–2024 papers had 30 questions a subject, the others 25.
Inside the chapter
Topics asked most.
- Mole concept64
- Stoichiometry58
- Empirical and molecular formula9
- Laws of chemical combination2
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10 Some Basic Concepts of Chemistry questions to practise.
Our own questions, written in the JEE Main pattern and checked, each with an explanation. Pick an option to see the answer.
Q1 · Chemistry · Some Basic Concepts of Chemistry
Ammonia has a fixed mass ratio of nitrogen to hydrogen of 14 : 3. What mass of nitrogen is present in an 8.5 g sample of pure ammonia?
Answer: (A)
By the law of definite proportions, a compound always contains its elements in the same mass ratio. Ammonia has N : H = 14 : 3 by mass, so total parts = 17. Nitrogen fraction = 14/17. For 8.5 g, nitrogen = 8.5 × 14/17 = 7.0 g.
Q2 · Chemistry · Some Basic Concepts of Chemistry
How many oxygen atoms are present in 0.10 mol of aluminium sulphate octadecahydrate, Al₂(SO₄)₃·18H₂O? (Avogadro constant = 6.022 × 10²³ mol⁻¹)
Answer: (C)
Count the oxygen atoms per formula unit: the three SO₄ groups give 3 × 4 = 12, and the 18 water molecules give 18, a total of 30. Moles of O atoms = 0.10 × 30 = 3.0 mol. Number of atoms = 3.0 × 6.022 × 10²³ ≈ 1.81 × 10²⁴.
Q3 · Chemistry · Some Basic Concepts of Chemistry
5.4 g of aluminium is heated with 14.2 g of chlorine gas: 2Al + 3Cl₂ → 2AlCl₃. What mass of aluminium is left unreacted when the reaction is over? (Al = 27, Cl = 35.5)
Answer: (B)
Moles of Al = 5.4/27 = 0.2 mol and moles of Cl₂ = 14.2/71 = 0.2 mol. 0.2 mol of Al would need 0.3 mol of Cl₂, so chlorine is the limiting reagent. It consumes 0.2 × 2/3 = 0.133 mol of Al, which is 3.6 g, leaving 5.4 − 3.6 = 1.8 g of aluminium.
Q4 · Chemistry · Some Basic Concepts of Chemistry
What volume, in litres, is occupied by 8 g of oxygen gas (O₂) at STP? (O = 16 g mol⁻¹; molar volume of an ideal gas at STP = 22.4 L mol⁻¹)
Answer: (A)
Principle: at STP, one mole of any ideal gas occupies 22.4 L, and moles are found by dividing the given mass by the molar mass of the substance as it actually exists. Oxygen gas is O₂, so its molar mass is 2 × 16 = 32 g mol⁻¹. Step 1: moles = 8 g ÷ 32 g mol⁻¹ = 0.25 mol. Step 2: volume = 0.25 mol × 22.4 L mol⁻¹ = 5.6 L. Always convert mass to moles before using the molar volume.
Q5 · Chemistry · Some Basic Concepts of Chemistry
50 mL of 0.20 M AgNO₃ solution is mixed with 40 mL of 0.10 M CaCl₂ solution. What mass of AgCl is precipitated? (Ag = 108, Cl = 35.5)
Answer: (C)
The reaction is Ag⁺ + Cl⁻ → AgCl, with 1 : 1 stoichiometry. Moles of Ag⁺ = 0.20 × 0.050 = 0.010. Moles of CaCl₂ = 0.10 × 0.040 = 0.0040, which gives 0.0080 mol of Cl⁻. Cl⁻ is limiting, so 0.0080 mol of AgCl forms. Mass = 0.0080 × 143.5 = 1.148 g ≈ 1.15 g.
Q6 · Chemistry · Some Basic Concepts of Chemistry
20 mL of a mixture of carbon monoxide and hydrogen is mixed with 20 mL of oxygen and exploded. The vessel is cooled to the initial temperature, at which water is a liquid, and all volumes are measured at the same temperature and pressure. The remaining gas measures 22 mL. The volume of carbon monoxide in the original mixture was:
Answer: (A)
By Gay-Lussac's law, gases react in simple volume ratios at the same temperature and pressure. The reactions are 2CO + O2 → 2CO2 and 2H2 + O2 → 2H2O(l). Each volume of either fuel needs 1/2 volume of O2, so 20 mL of mixture uses 10 mL of O2 whatever the composition, and 10 mL of O2 is left. Liquid water has negligible volume, so the only product gas is CO2, whose volume equals the volume of CO. The remaining gas is CO2 + leftover O2 = a + 10 = 22, so a = 12 mL.
Q7 · Chemistry · Some Basic Concepts of Chemistry
The empirical formula of a compound is CH2O and its molar mass is 180 g/mol. What is its molecular formula? (C = 12, H = 1, O = 16)
Answer: (A)
Step 1: Compute the empirical formula mass of CH2O: 12 + 2(1) + 16 = 30 g/mol. Step 2: Find n = molar mass / empirical formula mass = 180 / 30 = 6. Step 3: Multiply every subscript in the empirical formula by 6, giving C6H12O6.
Q8 · Chemistry · Some Basic Concepts of Chemistry
When 1.35 g of aluminium is burnt completely in excess oxygen, 2.55 g of aluminium oxide is obtained. What is the empirical formula of the oxide? (Al = 27, O = 16)
Answer: (D)
The mass of oxygen combined is 2.55 − 1.35 = 1.20 g. Moles of Al = 1.35/27 = 0.050 and moles of O = 1.20/16 = 0.075. The ratio Al:O = 0.050:0.075 = 2:3, so the empirical formula is Al₂O₃.
Q9 · Chemistry · Some Basic Concepts of Chemistry
Elements X and Y combine to form a single compound in which, by mass, 3 g of X are always present for every 8 g of Y. When 6.0 g of X and 10.0 g of Y are made to react completely to the extent possible, what is the mass of the compound formed and which reactant is left over?
Answer: (C)
By the law of definite proportions the compound always has X : Y = 3 : 8 by mass, so the reactants combine only in that ratio. 6.0 g of X would need 6 × 8/3 = 16 g of Y, but only 10.0 g is available. Y is therefore the limiting reactant. 10.0 g of Y combines with 10 × 3/8 = 3.75 g of X. Compound formed = 3.75 + 10.0 = 13.75 g. X left over = 6.0 − 3.75 = 2.25 g. Check: 13.75 + 2.25 = 16.0 g, which equals the total starting mass (conservation of mass).
Q10 · Chemistry · Some Basic Concepts of Chemistry
In a closed vessel, 2.00 g of substance X reacts completely with 3.00 g of substance Y. According to the law of conservation of mass, what is the total mass of product formed?
Answer: (A)
The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction. In a closed vessel the total mass of the products therefore equals the total mass of the reactants. Here 2.00 g + 3.00 g = 5.00 g.
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All 133 real JEE Main questions from Some Basic Concepts of Chemistry.
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About Some Basic Concepts of Chemistry in JEE Main.
How many questions come from Some Basic Concepts of Chemistry in JEE Main?
Some Basic Concepts of Chemistry (Chemistry, Class 11) has been asked 133 times in the 111 JEE (Main) shift papers from 2020–2025: about 1.2 questions a paper, 5% of all Chemistry questions. It is the 9th most-asked of 21 Chemistry chapters. The 2025 shifts had 20 from it in all.
Which topics of Some Basic Concepts of Chemistry are asked most in JEE Main?
Mole concept (64), Stoichiometry (58), Empirical and molecular formula (9), Laws of chemical combination (2), counted over the papers from 2020–2025.
Is Some Basic Concepts of Chemistry important for JEE Main 2027?
It is in the syllabus in force, so it can be asked. By past papers it ranks 9 of 21 in Chemistry; use that to decide how much time it gets, not whether to study it.
Where can I solve Some Basic Concepts of Chemistry JEE Main previous year questions?
On Nikaalle, free: sign up and practise every past JEE Main question from Some Basic Concepts of Chemistry with an explanation for each, or sit the full papers timed. The ten questions on this page are our own, written in the same pattern.
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