AMOUNT OF SUBSTANCE

By the late 18th century the early chemists needed to account for the relationships between the masses of reacting substances that they observed in their experiments, e.g., whenever two elements combined together in a chemical reaction they did so in a definite, fixed proportion by mass: Fe:S ratio 7 g:4 g and Mg:O ratio 3 g:2 g, for instance. At this point accurate and reproducible measurements of mass had only become recently available with the advent of reliable balances. Up until then the Science of Chemistry arguably did not exist... So, the birth of modern chemistry occurred around this time, when many new elements were being discovered by Priestley, Scheele, Davy and others. In 1808, however, John Dalton was the first scientist to base an atomic theory on experiment, and he was responsible for introducing explanations of experimental results in terms of atoms - the word atom is derived from the... amount of substance, stochiometry, formulae used for chemical calculations