Chemists often need to measure the quantities of elements and compounds reacting. Stoichiometry means* measuring elements. *Atoms of different elements (and sometimes the same element too) have different masses, so we need to take that into account when calculating reacting masses. The 'big numbers' that we put in front of formulae in chemical equations are the *stoichiometric coefficients.* Stoichiometry involves the concept of the *mole.*

### Particulate matter and chemical change

Writing formulae and chemical equations is a fundamental skill of a chemist (it is also a universal language). You are probably familiar with balancing chemical equations and many of the key terms in this part of the course.

### Reacting masses and volumes

There is not a great deal of knowledge to learn in order to be confident with mole calculations, but you may well need lots of practice. There are only certain ways in which questions on stoichiometry are asked in exams.

### The mole concept

A mole is simply a number, specifically Avogadro's number (L) which is 6.02 ×1023. It is the number of atoms in exactly 12 g of pure carbon-12. Mole calculations are not difficult in terms of the mathematical functions that you need to carry out, but it