Some Basic Concepts of Chemistry — NEET Chemistry MCQs with answers

Some Basic Concepts of Chemistry is a Class 11 Chemistry chapter in the NEET (UG) syllabus. NEET720 has 891 reviewed practice questions on it, each with a quick answer and a step-by-step explanation. The 8 questions below are free and fixed, so you can bookmark this page; the full chapter, plus mistake tracking and spaced revision, is in the app.

Topics covered

Stoichiometry & limiting reagent · Empirical & molecular formula · Equivalent concept · Mole concept and Avogadro number · Stoichiometry and limiting reagent · Concentration terms · Empirical and molecular formula · Laws of chemical combination and equivalence · Limiting reagent · Molarity and density · Stoichiometric calculation · Equivalent weight · Molarity and normality · Dimensional analysis · Molecular mass · Mole fraction · Atomic and molecular mass · Mole-mass relationship · Molar volume · Density · Conceptual · Some Basic Concepts of Chemistry · Mole and Avogadro number · Number of moles · Avogadro number applications · Empirical and molecular formula · Limiting reagent · Percentage yield · Equivalent weight · Molarity and dilution · Percentage composition · Gas volume at STP · Assertion-reason on mole concept · Stoichiometric calculations · Volume-volume stoichiometry · Normality · Conceptual mole relationships · Statement-based mole concept · Molality · Stoichiometry with excess reagent

8 free Some Basic Concepts of Chemistry practice questions with answers

Choose an answer in your head before opening it. Each explanation says why the correct option is right and, where relevant, why the tempting wrong option is wrong.

1. Question 1 · medium · Concentration terms

   While preparing a standard 0.1 M oxalic acid solution, a student dissolves the weighed acid in about 100 mL of water in a beaker, transfers it to a 250 mL volumetric flask with rinsings, and then must:

   • A.add distilled water carefully up to the calibration mark
   • B.add exactly 250 mL of distilled water using a measuring cylinder
   • C.add exactly 150 mL of water to make the total 250 mL
   • D.fill the flask to the brim and stopper it
   Show answer and explanation

   Answer: A. add distilled water carefully up to the calibration mark

   A volumetric flask is calibrated to contain its stated volume at the etched mark; making up to the mark fixes the final solution volume at exactly 250 mL.

   Molarity is defined per litre of SOLUTION, so the final solution volume must be exactly 250 mL. This is achieved only by adding water up to the calibration mark of the volumetric flask. Adding a measured volume of water (options B and C) fails because some water was already used for dissolving and rinsing, and solute-water volumes are not strictly additive.

   Common mistake: Adding a fixed volume of water instead of making the solution up to the mark

2. Question 2 · easy · Laws of chemical combination and equivalence

   The law of constant (definite) proportions states that:

   • A.Equal volumes of gases under identical conditions contain equal numbers of molecules
   • B.The mass of reactants equals the mass of products in a chemical reaction
   • C.When two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in a simple whole-number ratio
   • D.A given compound always contains exactly the same elements in the same proportion by mass, regardless of its source or method of preparation
   Show answer and explanation

   Answer: D. A given compound always contains exactly the same elements in the same proportion by mass, regardless of its source or method of preparation

   The law of definite proportions (Proust) says a pure compound has a fixed elemental composition by mass, independent of its origin — e.g., water is always 11.1% H and 88.9% O by mass.

   Proust's law of definite (constant) proportions: a chemical compound always contains the same elements combined together in the same fixed proportion by mass, irrespective of the source of the compound or the method used to prepare it. Option B is the law of conservation of mass (Lavoisier); option C is the law of multiple proportions (Dalton); option D is Avogadro's law. Only A correctly states definite proportions.

   Common mistake: Confusing the law of definite proportions with conservation of mass or multiple proportions

3. Question 3 · easy · Laws of chemical combination and equivalence

   The number of significant figures in the measurement 0.00620 kg is:

   • A.5
   • B.6
   • C.2
   • D.3
   Show answer and explanation

   Answer: D. 3

   Leading zeros before the first non-zero digit are never significant; trailing zeros after a decimal point are significant. 0.00620 → significant digits are 6, 2, 0 → 3 significant figures.

   Rules: (i) all non-zero digits are significant; (ii) zeros between non-zero digits are significant; (iii) leading zeros (before the first non-zero digit) are NOT significant — they only locate the decimal point; (iv) trailing zeros after a decimal point ARE significant. In 0.00620, the zeros before '6' are leading zeros (not significant); '6' and '2' are significant; the trailing '0' after '2' is significant (rule iv). Total significant figures = 3 (6, 2, 0).

   Common mistake: Counting leading zeros as significant or ignoring a significant trailing zero

4. Question 4 · easy · Laws of chemical combination and equivalence

   The law of conservation of mass states that:

   • A.The total number of moles of gas remains constant in any reaction
   • B.The mass of a compound is always a whole-number multiple of the masses of its constituent elements
   • C.Elements combine in a fixed ratio by mass to form a given compound
   • D.Mass can neither be created nor destroyed in a chemical reaction; total mass of reactants equals total mass of products
   Show answer and explanation

   Answer: D. Mass can neither be created nor destroyed in a chemical reaction; total mass of reactants equals total mass of products

   Lavoisier's law of conservation of mass: in any chemical change, the total mass of the system remains constant — mass of reactants = mass of products.

   The law of conservation of mass (Lavoisier, 1789) states that matter can neither be created nor destroyed in a chemical reaction, so the total mass of reactants must equal the total mass of products, provided the system is closed. It is a foundational law verified by careful mass balance experiments and underlies the practice of balancing chemical equations.

   Common mistake: Confusing conservation of mass with definite proportions or an invented mole-conservation rule

5. Question 5 · easy · Laws of chemical combination and equivalence

   Avogadro's law states that, at the same temperature and pressure:

   • A.Equal volumes of all gases have equal masses
   • B.Equal masses of all gases contain equal numbers of molecules
   • C.Equal volumes of all gases contain equal numbers of molecules
   • D.Equal numbers of moles of all gases occupy different volumes depending on their molar mass
   Show answer and explanation

   Answer: C. Equal volumes of all gases contain equal numbers of molecules

   Avogadro's law: at constant T and P, equal volumes of any gas contain equal numbers of molecules (and hence equal moles), regardless of the gas's identity.

   Avogadro's hypothesis/law states that under identical conditions of temperature and pressure, equal volumes of all gases (ideal behaviour assumed) contain equal numbers of molecules. This is independent of the chemical nature or molar mass of the gas. It follows that equal volumes contain equal moles, which underlies molar volume (22.4 L at STP) and Gay-Lussac's volume-ratio observations for reacting gases.

   Common mistake: Substituting 'mass' for 'number of molecules' in the law's statement

6. Question 6 · easy · Laws of chemical combination and equivalence

   Which of the following is an exact number, and therefore treated as having infinite significant figures in a calculation?

   • A.The number of atoms in one mole (Avogadro's number), or a small counted number like '2 molecules of water'
   • B.A mass measured on a digital balance, such as 5.20 g
   • C.A volume read from a burette, such as 24.6 mL
   • D.A temperature reading from a thermometer, such as 298 K
   Show answer and explanation

   Answer: A. The number of atoms in one mole (Avogadro's number), or a small counted number like '2 molecules of water'

   Defined constants (Avogadro's number, conversion factors) and counted whole quantities are exact numbers with unlimited significant figures; they never limit the precision of a calculation. Measured quantities (mass, volume, temperature) always carry finite significant figures.

   Exact numbers arise from definitions (1 dozen = 12) or from counting discrete objects (2 molecules), and are considered to have an infinite number of significant figures — they never limit the precision of a calculated result. In contrast, any quantity obtained by measurement (mass on a balance, volume from a burette, temperature from a thermometer) is inherently uncertain in its last recorded digit and has a definite, finite number of significant figures.

   Common mistake: Treating a measured quantity as an exact number

7. Question 7 · easy · Stoichiometry and limiting reagent

   In a chemical reaction carried out with a non-stoichiometric mixture of reactants, the amount of product actually formed is decided by the

   • A.reactant present in the largest mass
   • B.reactant with the highest molar mass
   • C.reactant that is completely consumed first
   • D.reactant taken in excess
   Show answer and explanation

   Answer: C. reactant that is completely consumed first

   The limiting reagent is the reactant that is used up first; once it is exhausted the reaction stops, so it fixes the maximum product formed.

   When reactants are not in the exact stoichiometric ratio, one of them gets consumed completely before the others. This reactant is called the limiting reagent. Since no further reaction is possible after it is exhausted, the theoretical yield of every product is calculated from the moles of the limiting reagent, not from the reagent in excess, the heaviest reagent, or the one with the largest mass.

   Common mistake: Choosing the reactant with the smaller mass (or larger mass) instead of comparing moles against coefficients.

8. Question 8 · medium · Stoichiometry and limiting reagent

   Equal volumes of HCl and NaOH solutions of the same molarity are mixed. Which statement about the resulting solution is correct?

   • A.It is acidic, because HCl is a strong acid and always dominates
   • B.It is neutral, because moles of acid and base are exactly equal for this 1 : 1 reaction
   • C.It is basic, because NaOH is a stronger electrolyte than HCl
   • D.Its nature cannot be predicted without knowing the actual molarity
   Show answer and explanation

   Answer: B. It is neutral, because moles of acid and base are exactly equal for this 1 : 1 reaction

   Equal volume × equal molarity gives equal moles; HCl and NaOH react 1 : 1 to give NaCl, whose solution is neutral.

   Moles of each = M × V and both products M and V are equal, so the acid and base are in the exact 1 : 1 stoichiometric ratio. Complete neutralization gives NaCl, the salt of a strong acid and strong base, which does not hydrolyse; the solution is neutral regardless of the common molarity chosen.

   Common mistake: Bringing acid/base 'strength' into a pure mole-balance question.

Questions about Some Basic Concepts of Chemistry for NEET

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