Mole Day: October 23

The mole (symbol: mol) is a unit of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity. The name "mole" was coined in German (as Mol) by Wilhelm Ostwald in 1893, although the related concept of equivalent mass had been in use at least a century earlier. The name is assumed to be derived from the word Molekül (molecule). The first usage in English dates from 1897, in a work translated from German.Some sources place the date of first usage in English as 1902. Merriam?Webster proposes an etymology from Molekulärgewicht (molecular weight). The names gram-atom and gram-molecule have also been used in the same sense as "mole", but these names are now obsolete.

The mole is defined as the amount of substance of a system that contains as many "elementary entities" (e.g. atoms, molecules, ions, electrons) as there are atoms in 12 g of carbon-12 (12C). A mole has . atoms or molecules of the pure substance being measured. A mole will possess mass exactly equal to the substance's molecular/atomic weight in grams. Because of this, one can measure the number of moles in a pure substance by weighing it and comparing the result to its molecular/atomic weight.

The current definition of the mole was approved during the 1960s: Prior to that, there had been definitions based on the atomic weight of hydrogen (about one gram of hydrogen-1 gas, excluding its heavy isotopes), the atomic weight of oxygen, and the relative atomic mass of oxygen-16: the four different definitions are equivalent to within 1%.

The most common method of measuring an amount of substance is to measure its mass and then to divide by the molar mass of the substance.International Bureau of Weights and Measures. "Realising the mole." Retrieved 25 September 2008. Molar masses may be easily calculated from tabulated values of atomic weights and the molar mass constant (which has a convenient defined value of 1 g/mol). Other methods include the use of the molar volume or the measurement of electric charge.

The Avogadro constant (symbols: L, NA) is the number of "elementary entities" (usually atoms or molecules) in one mole, that is (from the definition of the mole), the number of atoms in exactly 12 grams of carbon-12.. It was originally called 'Avogadros number'. The 2006 CODATA recommended value is:

:N_{\rm A}=6.022\ 141\ 79(30)\times 10^{23}\ \mbox{mol}^{-1}

The Avogadro constant is named after the early nineteenth century Italian scientist Amedeo Avogadro, who, in 1811, first proposed that the volume of a gas (at a given pressure and temperature) is proportional to the number of atoms or molecules regardless of the nature of the gas. English translation. The French physicist Jean Perrin in 1909 proposed naming the constant in honour of Avogadro. Extract in English, translation by Frederick Soddy. Perrin would win the 1926 Nobel Prize in Physics, in a large part for his work in determining the Avogadro constant by several different methods.Oseen, C.W. (December 10, 1926). Presentation Speech for the 1926 Nobel Prize in Physics.

The value of the Avogadro constant was first indicated by Johann Josef Loschmidt who, in 1865, estimated the average diameter of the molecules in air by a method that is equivalent to calculating the number of particles in a given volume of gas. English translation. This latter value, the number density of particles in an ideal gas, is now called the Loschmidt constant in his honour, and is approximately proportional to the Avogadro constant. The connection with Loschmidt is the root of the symbol L'' sometimes used for the Avogadro constant, and German language literature may refer to both constants by the same name, distinguished only by the units of measurement.