|Definition||The mole is defined as |
|Defining event||2018 |
|Effective since||Beginning 20 May 2019|
|Definition||The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 14076×1023 elementary entities. This number is the fixed numerical value of the 6.022|
The mole is the
The mole is widely used in chemistry as a convenient way to express amounts of reactants and products of chemical reactions. For example, the chemical equation 2H2 + O2 → 2H2O can be interpreted to mean that 2 mol
The term gram-molecule was formerly used for essentially the same concept. The term gram-atom has been used for a related but distinct concept, namely a quantity of a substance that contains Avogadro's number of atoms, whether isolated or combined in molecules. Thus, for example, 1 mole of MgBr2 is 1 gram-molecule of MgBr2 but 3 gram-atoms of MgBr2.
Thus, by definition, one mole of pure 12C has a mass of exactly 12
However, on 16 November 2018, after a meeting of scientists from more than 60 countries at the General Conference on Weights and Measures in Versailles, France, organised by the International Bureau of Weights and Measures (BIPM), all SI Units will be defined as constants. This means that any SI Unit, including the mole, will not be defined using physical objects but rather they will be defined by constants that are, in their nature, exact. Such changes will officially come into effect on 20 May 2019.
For example, one mole of hydrogen atoms, will be defined as containing 14076×1023 of hydrogen atoms in it, weighing exactly 1.008 grams (same as the Ar of one hydrogen atom) 6.022.
One can determine the amount of a known substance, in moles, by dividing the sample's mass by the substance's molar mass. Other methods include the use of the
The mass of one mole of a substance depends not only on its
Since the definition of the gram is not (as of 2011) mathematically tied to that of the atomic mass unit, the number of molecules per mole NA (the Avogadro constant) must be determined experimentally. The value adopted by
Mass and volume (properties of matter) are often used to quantify a sample of a substance. However, the volume changes with temperature and pressure. Similarly, due to relativistic effects, the mass of a sample changes with temperature, speed or gravity. This effect is very small at low temperature, speed or gravity, but at high speed like in a particle accelerator or theoretical space craft, the change is significant. The amount of substance remains the same regardless of temperature, pressure, speed or gravity, unless a (chemical or nuclear) reaction changes the number of particles.