Important Questions: CBSE Class 11 Chemistry Chapter 1 Some Basic Concepts of Chemistry

Chapter 1 'Some Basic Concepts of Chemistry' from NCERT Class 11 Chemistry typically contributes 5 to 8 marks in the annual CBSE board examination. The chapter covers the mole concept, stoichiometry, laws of chemical combination, empirical and molecular formulae, percentage composition, and significant figures. Recent CBSE question papers show that 2-3 MCQs (1 mark each) come from definitions and basic concepts, one 2-mark question on molar mass or Avogadro's number, and at least one 3-mark or 5-mark numerical on stoichiometry or limiting reagent. The 2023 board paper featured a 5-mark case-based question linking mole concept with real-world scenarios. Understanding this distribution helps students prioritise high-yield topics during revision. The chapter also sets the foundation for physical chemistry throughout Classes 11 and 12, so strong conceptual clarity here pays dividends in chapters on equilibrium, thermodynamics, and redox reactions.

CBSE question setters follow a predictable pattern when designing questions from Chapter 1. They test conceptual understanding through MCQs on definitions (matter, mixture, mole, significant figures), numerical ability via stoichiometry and mole-concept problems, and application skills through case-based or real-world scenario questions. In the 2024 sample paper, one MCQ asked students to identify the correct number of significant figures in a measured value, while a 3-mark question required calculating the empirical formula from percentage composition data. Five-mark questions often integrate multiple concepts: for instance, determining the limiting reagent in a reaction, calculating theoretical yield, and then computing percentage yield. CBSE also loves dimensional analysis questions where students must convert between grams, moles, and number of particles using Avogadro's constant. Understanding this blueprint allows you to practise strategically rather than randomly.

One-mark questions in CBSE Class 11 Chemistry Chapter 1 are designed to test recall and basic comprehension. MCQs appear in Section A of the board paper and cover definitions, SI units, significant figures, and simple factual knowledge. VSAs may ask you to write a formula, state a law, or perform a quick one-step calculation. Practising 6-8 representative MCQs sharpens speed and accuracy. Remember that negative marking applies to MCQs in the board exam, so educated guessing is better than wild guessing. Below are six important 1-mark questions with model answers that have appeared in various CBSE sample papers and previous board exams.

Two-mark questions require a brief explanation or a straightforward calculation with one or two steps. CBSE typically asks for molar mass calculations, simple mole-mass conversions, or application of Avogadro's number. Your answer should be concise—state the formula, plug in the values, and write the final answer with correct units. Marks are awarded for method (1 mark) and final answer (1 mark), so even if your arithmetic slips, showing the correct approach earns partial credit. Below are four high-probability 2-mark questions drawn from NCERT exemplar problems and past CBSE papers. Practise writing answers within three to four lines, maintaining clarity and logical flow.

Three-mark questions form the backbone of CBSE Chemistry numericals. They test your ability to apply formulas, perform multi-step calculations, and present working in a logical sequence. Common themes include empirical formula determination from percentage composition, stoichiometric calculations involving balanced equations, and percentage yield computations. CBSE mark schemes award 1 mark for correct formula/method, 1 mark for substitution with units, and 1 mark for final answer. Always write units at every step and round off according to significant figures. Below are four representative 3-mark questions that mirror the difficulty and style of recent board papers. Practise writing full solutions within half a page.

Five-mark questions are comprehensive problems that integrate multiple concepts from Chapter 1. CBSE introduced case-based questions from 2021 onwards, where a short paragraph describes a real-world scenario (industrial synthesis, environmental chemistry, nutritional analysis) followed by sub-questions. A typical 5-mark numerical might ask you to balance an equation, identify the limiting reagent, calculate theoretical yield, and then compute percentage yield or excess reagent left. These questions demand clarity in presentation, correct use of significant figures, and logical step-wise working. Marks are distributed as 1 + 1 + 1 + 1 + 1 across sub-parts or method steps. Below are three high-value 5-mark questions that closely resemble those in recent CBSE board papers. Allocate seven to eight minutes per question during practice.

Even strong students stumble in Chapter 1 due to avoidable errors. The most frequent mistake is incorrect molar mass calculation—forgetting to multiply the atomic mass by the subscript or using outdated atomic weights. Another pitfall is misapplying Avogadro's number: students often confuse 6.022 × 10²³ particles per mole with per gram. Significant figures cause marks loss when final answers are not rounded correctly; CBSE mark schemes explicitly deduct for wrong sig figs. In stoichiometry, many skip balancing the equation first or misread coefficients, leading to wrong mole ratios. Limiting reagent problems trip up students who do not compare mole ratios with stoichiometric coefficients. Unit conversion errors, especially between grams and kilograms or litres and millilitres, are common. Finally, students often write the empirical formula as the molecular formula without checking the molar mass ratio. Being aware of these pitfalls and double-checking each step prevents silly marks loss.

Stoichiometry is the heart of Chapter 1 and one of the highest-weightage topics in CBSE Class 11 Chemistry. Questions on stoichiometry test your ability to use balanced chemical equations to relate masses, moles, and volumes of reactants and products. The limiting reagent concept appears almost every year in 3-mark or 5-mark questions. The trick is to convert all given masses to moles, divide by stoichiometric coefficients, and identify which reagent produces the least product. That reagent is limiting; the others are in excess. After identifying the limiting reagent, use it to calculate the theoretical yield of product. Finally, if actual yield is given, compute percentage yield as (actual/theoretical) × 100. CBSE loves multi-step problems that combine these elements. Practising the four questions below will make you exam-ready in this crucial area.

Empirical and molecular formula questions are staples in CBSE papers. The empirical formula represents the simplest whole-number ratio of atoms, while the molecular formula shows the actual number of atoms in a molecule. To find the empirical formula from percentage composition, assume a 100 g sample, convert percentages to grams, then to moles using atomic masses, and divide by the smallest number of moles to get the simplest ratio. If the ratio is not whole, multiply by a small integer (2, 3, or 4) to clear fractions. To find the molecular formula, calculate the empirical formula mass and divide the given molar mass by it to get n, then multiply each subscript in the empirical formula by n. CBSE often provides combustion data or elemental analysis data instead of direct percentages, requiring you to back-calculate the masses of C, H, and O. The three questions below cover all these variations.

Percentage composition questions ask you to find the mass percent of each element in a compound, or vice versa. The formula is straightforward: (mass of element in 1 mole / molar mass of compound) × 100. CBSE also tests purity and percentage yield concepts. Purity is the ratio of pure substance mass to total sample mass, expressed as a percentage. Percentage yield compares actual experimental yield with theoretical yield calculated from stoichiometry. These questions often appear in 2-mark or 3-mark formats and are easy scoring if you remember the formulas and show clear working. Below are sample problems that illustrate each variation. Make sure you write the formula first, substitute values with units, and box the final answer.

Chapter 1 numericals demand clarity in concept and speed in calculation—both of which come from repeated practice with instant feedback. CBSETUTOR.ai offers a 24×7 AI tutor that acts like a personal chemistry coach for CBSE Class 11 students. Stuck on a stoichiometry problem at 11 p.m.? Simply snap a photo of the question and upload it; the AI breaks down the solution step-by-step, highlighting the formula, showing substitution with units, and explaining common mistakes. Unlike generic video lectures, CBSETUTOR.ai tailors explanations to your exact doubt. It covers all NCERT exercises, exemplar problems, and previous years' board questions for Chapter 1. Parents across Delhi NCR, Mumbai, and Bengaluru trust CBSETUTOR.ai because it costs just ₹999 per month for full access across Classes 6 to 12—one flat fee, no hidden charges. A 3-day free trial lets your child experience the platform risk-free. For Chapter 1, the AI also generates unlimited practice questions on mole concept, empirical formula, and limiting reagent, so students build speed and confidence before the exam.

Chapter 1 is concept-heavy and calculation-intensive, so a focused revision strategy is essential. Start by reviewing NCERT in-text solved examples and end-of-chapter exercises; these form the blueprint for CBSE questions. Make a one-page formula sheet covering molar mass, Avogadro's number, percentage composition, empirical and molecular formula steps, stoichiometry ratios, and limiting reagent logic. Practise at least 15-18 numerical problems of varying difficulty—6 easy, 6 medium, 6 hard—to build speed and accuracy. Time yourself: 1-mark MCQs should take 30 seconds, 2-mark questions 2 minutes, 3-mark questions 4 minutes, and 5-mark questions 7 minutes. Rework any problem you get wrong until the method is automatic. Pay special attention to significant figures and unit consistency; these are easy marks if you are careful. One day before the exam, solve a full Chapter 1 question set in exam conditions, then review mistakes. This active recall strategy cements memory better than passive rereading. CBSETUTOR.ai's topic tests simulate real exam pressure and provide instant performance analytics.