ABSTRACT
Many biological systems are concerned with the action of a chemical substance upon a particular process. Furthermore, these chemicals (solutes) are often dispersed in a suitable solvent such as water to form a solution. To quantify the actions of such solutes the amount in solution needs to be specified in a way which allows comparison between different solutions, and the most useful concept in this context is the mole. One mole of any substance contains the same number of particles (atoms, molecules, etc.). To define this number a ‘gold standard’ is required which is at present the common isotope of carbon, 12C. Therefore one mole of a substance contains the same number of particles as there are atoms in exactly 0.012 kilogrammes (12g) of 12C; this number, called Avogadro’s constant, NA, is about 6.022×1023.
