Class 11 Chemistry Chapter 1 Some Basic Concepts of Chemistry – Revision Notes

Chapter 1, Some Basic Concepts of Chemistry, establishes the laws of chemical combination and the mole concept, which together form the basis for every quantitative calculation in chemistry, from balancing equations to determining formulas.

Last Updated: September 23, 2026

Key Concepts

The laws of chemical combination — conservation of mass, constant proportions and multiple proportions — describe how elements combine to form compounds in fixed, predictable ratios. The mole concept links the microscopic and macroscopic worlds: 1 mole of any substance contains 6.022×10²³ particles (Avogadro’s number, N_A), and the number of moles (n) equals mass divided by molar mass, or the number of particles divided by N_A. Concentration can be expressed in several ways: molarity (moles of solute per litre of solution), mass percentage (mass of solute divided by mass of solution, times 100), and mole fraction (moles of one component divided by total moles). In a chemical reaction, the limiting reagent is the reactant that gets used up first, and it determines the maximum amount of product that can form.

Quick Recap

• 1 mole = 6.022×10²³ particles (Avogadro’s number).
• n (moles) = mass ÷ molar mass = number of particles ÷ N_A.
• Molarity = moles of solute ÷ litres of solution.
• The limiting reagent runs out first and caps how much product a reaction can produce, regardless of how much of the other reactant remains.

Common Mistake to Avoid

Students often assume the reactant present in the largest quantity is automatically the limiting reagent. The limiting reagent is decided by the reaction’s mole ratio, not by which reactant has the greater mass or volume; a reactant present in a large amount can still run out first if the reaction needs much more of it per mole of product.

Frequently Asked Questions

Q. Why is the mole concept so central to chemistry calculations?
Because atoms and molecules are far too small to count or weigh individually, the mole gives chemists a practical, countable unit that connects a substance’s mass directly to the actual number of particles it contains.

Full walkthrough: Chapter 1 Solutions · Extra Questions.

Written by Satish

NCERTBooks.org is an independent educational resource run by a small team focused on making official NCERT textbooks easy to find, read, and download for students, parents, and teachers across India. We are not affiliated with NCERT or the Ministry of Education -- we organise publicly available NCERT content by class and subject, verify links against official sources, and build tools (like our in-browser reader) that make studying more convenient. Every guide we publish is written and reviewed by our team based on the actual NCERT curriculum and syllabus.