Introduction

This topic on stoichiometry is based on three essential ideas.  If you have studied some chemistry before you will probably already know the first - that the physical and chemical properties of substances depend on the ways in which different atoms combine. The second one states that the concept of a mole makes it possible to correlate the (extremely large) number of particles with the mass of the substance that can be measured. The third essential idea is that, by applying Avogadro's law, mole ratios in chemical equations can be used to calculate reacting ratios by mass and gas volume.

In some ways it makes sense to put stoichiometric relationships as the first topic on the syllabus. In the early years of chemistry the formulas of substances were first determined by quantitative analysis. The quantitative work of Lavoisier and others helped to make chemistry a modern scientific subject and moved it away from alchemy. However now that an understanding of chemistry is firmly based on atomic theory many teachers think that it makes sense to learn about the structure of the atom (Topic 2) before tackling Topic 1. This website follows the order given in the IB syllabus but it may well not be the order in which your teacher teaches it.

Many students will put 'Stoichiometry' high on their list of topics that  they find most difficult in IB Chemistry. It is an important topic as chemical calculations and the concepts behind them are required in many of the other topics such as rates of reaction, acids and bases and redox chemistry. What this does mean is that throughout the course there will be a steady 'drip feed' of questions and practical assignments involving stoichiometry so that even if you find it difficult at first you will gradually increase in confidence and understanding as the course proceeds.