1.1: Units
There is international agreement that the units used for physical quantities in science and technology should be those of the International System of Units, or SI (standing for the French Système International d’Unités ). The Physical Chemistry Division of the International Union of Pure and Applied Chemistry, or IUPAC, produces a manual of recommended symbols and terminology for physical quantities and units based on the SI. The manual has become known as the Green Book (from the color of its cover) and is referred to here as the IUPAC Green Book. This e-book will, with a few exceptions, use symbols recommended in the third edition (2007) of the IUPAC Green Book (E. Richard Cohen et al, Quantities, Units and Symbols in Physical Chemistry , 3rd edition. RSC Publishing, Cambridge, 2007). These symbols are listed for convenient reference in Appendices C and D.
Any of the symbols for units listed in Tables 1.1–1.3, except kg and \(\degC\), may be preceded by one of the prefix symbols of Table 1.4 to construct a decimal fraction or multiple of the unit. (The symbol g may be preceded by a prefix symbol to construct a fraction or multiple of the gram.) The combination of prefix symbol and unit symbol is taken as a new symbol that can be raised to a power without using parentheses, as in the following examples:
-
The physical quantity formally called
amount of substance
is a counting quantity for particles, such as atoms or molecules, or for other chemical entities. The counting unit is invariably the
mole
, defined as the amount of substance containing as many particles as the number of atoms in exactly \(12\) grams of pure carbon-12 nuclide, \({}^{12}\)C. See Appendix A for the wording of the official IUPAC definition. This definition is such that one mole of H\(_{2}\)O molecules, for example, has a mass of \(18.0153\) grams (where \(18.0153\) is the relative molecular mass of H\(_{2}\)O) and contains \(6.02214\timesten{23}\) molecules (where \(6.02214 \timesten{23}\units{mol\(^{-1}\)}\) is the
Avogadro constant
to six significant digits). The same statement can be made for any other substance if \(18.0153\) is replaced by the appropriate atomic mass or molecular mass value.
The symbol for amount of substance is \(n\). It is admittedly awkward to refer to \(n\)(H\(_2\)O) as “the amount of substance of water.” This e-book simply shortens “amount of substance” to amount . An alternative name suggested for \(n\) is “chemical amount.” Thus, “the amount of water in the system” refers not to the mass or volume of water, but to the number of H\(_2\)O molecules in the system expressed in a counting unit such as the mole.