1.6 Mole

Main article: Mole (unit)
A mole is the amount of a substance that contains as many elementary entities (atoms, molecules or ions) as there are atoms in 0.012 kilogram (or 12 grams) of carbon-12, where the carbon-12 atoms are unbound, at rest and in their ground state.[38] This number is known as the Avogadro constant, and is determined empirically. The currently accepted value is 6.02214179(30) × 1023 mol−1 (2007 CODATA). The best way to understand the meaning of the term "mole" is to compare it to terms such as dozen. Just as one dozen is equal to 12, one mole is equal to 6.02214179(30) × 1023. The term is used because it is much easier to say, for example, 1 mole of carbon atoms, than it is to say 6.02214179(30) × 1023 carbon atoms. Likewise, we can describe the number of entities as a multiple or fraction of 1 mole, e.g. 2 mole or 0.5 moles. Mole is an absolute number (having no units) and can describe any type of elementary object, although the mole's use is usually limited to measurement of subatomic, atomic, and molecular structures.
The number of moles of a substance in one liter of a solution is known as its molarity. Molarity is the common unit used to express the concentration of a solution in physical chemistry

The mole (symbol mol) is the SI base unit[1] of amount of substance, one of a few units used to measure this physical quantity. The name "mole" is an 1897 translation[2][3] of the German Mol, coined by the chemist Wilhelm Ostwald in 1893,[4] 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 German word Molekül (molecule).

The mole is commonly used in titration, a laboratory method in chemistry to determine the concentration of some substance in a solution. In this context, millimoles per litre (mmol/L), micromoles/litre (µmol/L), or nanomoles/L (nmol/L) are often used.

For pure substances (that is, not being an admixture of different substances) the mole is defined as the amount of substance that contains as many "elementary entities" (e.g. atoms, molecules, ions, electrons) as there are atoms in 12 gram of carbon-12 (12C). Thus, by definition, one mole of pure 12C has a mass of exactly 12 g. The number of atoms or molecules contained in one mole of a pure substance is known as the Avogadro constant (or Avogadro's number). By convention it has dimension mol−1, and its experimentally determined value is approximately 6.022142×1023 mol−1. So a mole of any pure substance has mass in grams exactly equal to that substance's molecular or atomic mass; e.g., 1 mol of calcium-40 is equal to 40 g approximately (because Ca-40 has a mass of 39.9625906 amu on the C-12 scale). In other words, the numerical value of a substance's molecular or atomic mass in atomic mass units is the same as that of its molar mass (the mass of one mole of that substance) in grams. (Although the SI base unit for mass is the kilogram, for practical and historical reasons grams are commonly used in this context, especially in chemistry.)

The most common method of determining the amount, expressed in moles, of pure substance the value of whose molar mass is known, is to measure its mass in grams and then to divide by its molar mass (expressed in g/mol). 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 current definition of the mole was approved during the 1960s.[1][8] Earlier definitions had been based on the atomic mass 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 were equivalent to within 1%.

The names gram-atom (abbreviated gat.) and gram-molecule have also been used in the same sense as "mole"[1]. However, modern conventions define the gram-atom and the mole differently. While the elementary entity defining a mole will vary depending on the substance, the elementary entity for the gram-atom is always the atom. For example, 1 mole of He is equivalent to 1 gram-atom of He, but 1 mole of MgB2 is equivalent to 3 gram-atoms of MgB2.