Chapter 1 · Introduction

Chapter 1 — Some Basic Concepts of Chemistry

How chemists measure, count and weigh what they cannot see — units and significant figures, atoms and the mole, and the arithmetic of reactions.

A spoonful of water — about $18\ \mathrm{g}$ — contains roughly $6\times10^{23}$ water molecules. If you could count a million of them every second, the count would take about twenty thousand million years, longer than the age of the universe. Yet a chemist in a school laboratory "counts" them in a few seconds, with a balance. This chapter explains how: by measuring carefully, by knowing how much one atom weighs relative to another, and by grouping particles into a chemist's dozen called the mole.

These ideas are the grammar of everything that follows. Whenever a later chapter asks how much — how much energy a fuel releases, how much product a reaction can give, how acidic a solution is, how much permanganate a titration needs — the answer begins with the measurement habits of Section 1.1, the mole of Section 1.2 and the stoichiometry of Section 1.3. In JEE and NEET the "mole concept" appears, directly or hidden inside another topic, in almost every paper.

Matter & its kinds→SI units & significant figures→Laws of combination→Atomic & molar mass→The mole→Formulae & equations→Limiting reagent & concentration

🎯 By the end of this chapter you can…

classify matter, use SI units and prefixes, and convert units by dimensional analysis;
report every measured or calculated value with the correct number of significant figures, and tell precision from accuracy;
state and apply the laws of chemical combination, Dalton's atomic theory and Avogadro's law;
calculate average atomic masses, molecular and formula masses, and move freely between mass, moles and number of particles;
find percentage composition, empirical and molecular formulae, and carry out stoichiometric and limiting-reagent calculations;
express the concentration of a solution as mass percent, mole fraction, molarity or molality, and convert between them.

Choose a substance and a mass. Dividing by the molar mass $M$ gives the amount in moles; multiplying by $N_{A}=6.022\times10^{23}\ \mathrm{mol^{-1}}$ gives the number of particles. In the particle box each dot stands for $0.05\ \mathrm{mol}$ — that is $3.011\times10^{22}$ particles. Compare $18\ \mathrm{g}$ of water with $18\ \mathrm{g}$ of glucose.

Substance:mass (g):18

mass $m$ (g)—

molar mass $M$ ($\mathrm{g\,mol^{-1}}$)—

amount $n=m/M$ (mol)—

particles $N=nN_{A}$—

atoms—

Sections in this chapter

1.1

Matter, Measurement and Significant Figures

Kinds of matter, SI units, density and temperature, significant figures, precision versus accuracy and dimensional analysis.

$\rho=\dfrac{m}{V}$, $T(\mathrm{K})=t(^{\circ}\mathrm{C})+273.15$

Open section →1.2

Laws of Chemical Combination, Atomic Mass and the Mole

The five laws, Dalton's theory, average atomic mass from isotopes, molecular mass, the mole and molar mass.

$n=\dfrac{m}{M}=\dfrac{N}{N_{A}}$

Open section →1.3

Stoichiometry: Formulae, Limiting Reagents and Concentration

Percentage composition, empirical and molecular formulae, reaction calculations, the limiting reagent, and four ways to state concentration.

$M=\dfrac{n_{2}}{V_{\text{soln}}\,(\mathrm{L})}$, $M_{1}V_{1}=M_{2}V_{2}$

Open section →

✅ Before you start

You should be able to write and balance simple chemical equations, know the symbols and valencies of common elements and the formulae of common acids, bases and salts — all from Class 10 Chemistry. From mathematics you need ratios and proportion, rearranging a formula such as $\rho=m/V$, and powers of ten. If any of this feels rusty, try the prerequisite check on the welcome page first.

← Welcome to Class 11 Chemistry 1.1 Matter, Measurement and Significant Figures →