# Introduction

Atoms are small, they have incredibly small masses. Therefore weighing them out, if you want to carry out a chemical reaction or analytical procedure is not realistic. For example, you may wish to make up a solution having a particular concentration, in theory this involves measuring out billions and billions of molecules at a time so you know how many molecules of a substance it contains.

Alternatively you may wish to calculate the weights of reactants for a reaction in a ratio that matches the ratio of reactants in its chemical equation. For a general equation: X + Y = Z, you cannot simply take one gram of X, and one gram of Y, as they will contain different numbers of particles (be they atoms, molecules and ions).

This is where the concept of the mole is so important to the chemist, and as you will see one mole of any substance contains the same number of particles. However, as you will be dealing with quantities such as mass and volume throughout this section, your starting point is a look at how these are measured.

OpenLearn - Discovering chemistry
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