The mole (symbol: mol) is the SI base unit that measures an amount of substance. The mole is a counting unit. One mole contains Avogadro's number (approximately 6.02214×10²³) entities (atoms or molecules). “SI” redirects here. ... The amount of substance, n, of a sample or system is a physical quantity which is proportional to the number of elementary entities present. ... The Avogadro constant (symbols: L, NA), also called the Avogadro number and, in German scientific literature, sometimes also known as the Loschmidt constant/number, is formally defined to be the number of entities in one mole,[1][2] that is the number of carbon-12 atoms in 12 grams (0. ... The Avogadro constant (symbols: L, NA), also called the Avogadro number and, in German scientific literature, sometimes also known as the Loschmidt constant/number, is formally defined to be the number of entities in one mole,[1][2] that is the number of carbon-12 atoms in 12 grams (0. ...

A mole is much like "a dozen" in that both are absolute numbers (having no units) and can describe any type of elementary object (object made up of atoms). The mole's use, however, is usually limited to measurement of subatomic, atomic, and molecular structures; tradition and its magnitude compared to more common units make it impractical for other uses. Dozen is another word for the number twelve. ... Helium atom (not to scale) Showing two protons (red), two neutrons (green) and a probability cloud (gray) of two electrons (yellow). ... Properties For other meanings of Atom, see Atom (disambiguation). ... 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ...

In practice, one often measures an amount of the substance in a gram-mole, which is the quantity of a substance whose mass in grams is equal to its formula weight. Thus a gram-mole for Carbon-12 is 12 grams, while for water it is 18.016 grams. The entity counted is usually an atom (as in C) or a molecule (as in H₂O, molecular formula weight = 2 H atoms + 1 O atom ≈18). H2O is the chemical formula for Water (molecule). ...

Definitions

A mole is the amount of substance of a system, which contains as many elementary entities 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.^[1] The number of atoms in 0.012 kilogram of carbon-12 is known as the Avogadro constant, and is determined empirically. The currently accepted value is 6.02214279(30)×10²³ mol^-1 (2007 CODATA). The amount of substance, n, of a sample or system is a physical quantity which is proportional to the number of elementary entities present. ... Kg redirects here. ... BIC pen cap, about 1 gram. ... Carbon 12 is a stable isotope of the element carbon. ... In physics, the ground state of a quantum mechanical system is its lowest-energy state. ... The Avogadro constant (symbols: L, NA), also called the Avogadro number and, in German scientific literature, sometimes also known as the Loschmidt constant/number, is formally defined to be the number of entities in one mole,[1][2] that is the number of carbon-12 atoms in 12 grams (0. ... CODATA (Committee on Data for Science and Technology) was established in 1966 as an interdisciplinary committee of the International Council of Science (ICSU), formerly the International Council of Scientific Unions. ...

According to the SI, the mole is not dimensionless, but has its very own dimension, namely "amount of substance", comparable to other dimensions such as mass and luminous intensity.^[2] (By contrast, the SI specifically defines the radian and the steradian as special names for the dimensionless unit one.)^[3] The SI additionally defines the Avogadro constant as having the unit reciprocal mole, as it is the ratio of a dimensionless quantity and a quantity with the unit mole.^[3] However, if in the future the kilogram is redefined in terms of a specific number of carbon-12 atoms (see below), then the value of Avogadro's number will be defined rather than measured, and the mole will cease to be a unit of physical significance.^[4] In the physical sciences, a dimensionless number (or more precisely, a number with the dimensions of 1) is a quantity which describes a certain physical system and which is a pure number without any physical units; it does not change if one alters ones system of units of measurement... 2-dimensional renderings (ie. ... For other uses, see Mass (disambiguation). ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... Some common angles, measured in radians. ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ...

The relationship of the atomic mass unit (u^[5]) to Avogadro's number means that a mole can also be defined as: That quantity of a substance whose mass in grams is the same as its formula weight. For example, iron has a relative atomic mass of 55.845 u, so a mole of iron has a mass of 55.845 grams. This notation is very commonly used by chemists and physicists. The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic and molecular masses. ... ... For other uses, see Iron (disambiguation). ... In reference to a certain isotope of a chemical element, atomic mass (though also called relative atomic mass and atomic weight) is the mass of one atom of the isotope expressed in units (atomic mass unit, amu) such that the carbon-12 isotope has an atomic mass of exactly 12. ...

Scientists and engineers (chemical engineers in particular) sometimes measure amount of substance in units of gram-moles, kilogram-moles, pound-moles, or ounce-moles; these measure the quantity of a substance whose mass in grams, kilograms, pounds, or ounces (respectively) is equal to its formula weight. The SI mole is identical to the gram-mole.

Elementary entities

When the mole is used to specify the amount of a substance, the kind of elementary entities (particles) in the substance must be identified. The particles can be atoms, molecules, ions, formula units, electrons, photons or other particles. For example, one mole of water is equivalent to 18.016 grams of water and contains one mole of H₂O molecules, but three moles of atoms (two moles H and one mole O). Properties For other meanings of Atom, see Atom (disambiguation). ... 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... This article is about the electrically charged particle. ... A formula unit in chemistry is the empirical formula of an ionic or covalent network solid compound used as an independent entity for stoichiometric calculations. ... For other uses, see Electron (disambiguation). ... In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ...

When the substance of interest is a gas, the particles are usually molecules. However, the noble gases (He, Ar, Ne, Kr, Xe, Rn) are all monoatomic, that is each particle of gas is a single atom. An ideal gas has a molar volume of 22.4 litres per mole at STP (see Avogadro's Law). Gas phase particles (atoms, molecules, or ions) move around freely Gas is one of the four major states of matter, consisting of freely moving atoms or molecules without a definite shape and without a definite volume. ... This article is about the chemical series. ... An ideal gas or perfect gas is a hypothetical gas consisting of identical particles of zero volume, with no intermolecular forces. ... In chemistry, the molar volume of a substance is the ratio of the volume of a sample of that substance to the amount of substance (usually in mole) in the sample. ... The litre or liter (see spelling differences) is a unit of volume. ... Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ... This article does not cite any references or sources. ...

A mole of atoms or molecules is also called a "gram atom" or "gram molecule", respectively.

History

The name mole (German Mol) is attributed to Wilhelm Ostwald who introduced the concept in the year 1902. It is an abbreviation for molecule (German Molekül), which is in turn derived from Latin moles "mass, massive structure". He used it to express the gram molecular weight of a substance. So, for example, 1 mole of hydrochloric acid (HCl) has a mass of 36.5 grams (atomic masses Cl: 35.5 u, H: 1.0 u). Friedrich Wilhelm Ostwald (commonly just Wilhelm Ostwald) (September 2, 1853 - April 4, 1932) was a German chemist. ... 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic and molecular masses. ...

Prior to 1959 both the IUPAP and IUPAC used oxygen to define the mole, the chemists defining the mole as the number of atoms of oxygen which had mass 16 g, the physicists using a similar definition but with the oxygen-16 isotope only. The two organizations agreed in 1959/1960 to define the mole as such: The International Union of Pure and Applied Physics (IUPAP) is an international non-governmental organization devoted to the advancement of Physics. ... The International Union of Pure and Applied Chemistry (IUPAC) is an international non-governmental organization devoted to the advancement of chemistry. ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colourless (gas) colourless (liquid) Standard atomic weight 15. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance transparent (gas) very pale blue (liquid) Atomic mass 15. ...

The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12; its symbol is "mol."

This was adopted by the ICPM (International Committee for Weights and Measures) in 1967, and in 1971 it was adopted by the 14th GCPM (General Conference on Weights and Measures). Kg redirects here. ... The International Committee for Weights and Measures is the English name of the Comité international des poids et mesures (CIPM, sometimes written in English Comité International des Poids et Mesures). ...

In 1980 the ICPM clarified the above definition, defining that the carbon-12 atoms are unbound and in their ground state. In physics, the ground state of a quantum mechanical system is its lowest-energy state. ...

Proposed future definition

As with other SI base units, there have been proposals to redefine the kilogram in such a way as to define some presently measured physical constants to fixed values. One proposed definition of the kilogram is: Look up si, Si, SI in Wiktionary, the free dictionary. ... Kg redirects here. ... In science, a physical constant is a physical quantity whose numerical value does not change. ...

The kilogram is the mass of exactly (6.0221415×10²³/0.012) unbound carbon-12 atoms at rest and in their ground state. ^[6]

This would have the effect of defining Avogadro's number to be precisely N_A = 6.0221415×10²³ elementary entities per mole, and, consequently, the mole would become merely a unit of counting, like the dozen. Dozen is another word for the number twelve. ...

Another proposed definition of N_A is:

N_A = 602214141070409084099072 = 84446888³

This has the convenient properties of being a perfect cube, and of being near the current experimental bounds of measurement.^[7]

Utility of moles

The mole is useful in chemistry because it allows different substances to be measured in a comparable way. Using the same number of moles of two substances, both amounts have the same number of molecules or atoms. The mole makes it easier to interpret chemical equations in practical terms. Thus the equation: For other uses, see Chemistry (disambiguation). ... 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... Properties For other meanings of Atom, see Atom (disambiguation). ...

2H₂ + O₂ → 2H₂O

can be understood as "two moles of hydrogen plus one mole of oxygen yields two moles of water."

Moles are useful in chemical calculations, because they enable the calculation of yields and other values when dealing with particles of different mass.

Number of particles is a more useful unit in chemistry than mass or weight, because reactions take place between atoms (for example, two hydrogen atoms and one oxygen atom make one molecule of water) that have very different weights (one oxygen atom weighs almost 16 times as much as a hydrogen atom). However, the raw numbers of atoms in a reaction are not convenient, because they are very large; for example, just one mL of water contains over 6.02×10²² molecules. The millilitre is the equivalent of a cubic centimetre. ...

See also

The Avogadro constant (symbols: L, NA), also called the Avogadro number and, in German scientific literature, sometimes also known as the Loschmidt constant/number, is formally defined to be the number of entities in one mole,[1][2] that is the number of carbon-12 atoms in 12 grams (0. ... An einstein is a unit used in irradiance and in photochemistry. ... In physics, the faraday (not to be confused with the farad) is a unit of electrical charge; one faraday is equal to the charge of 6. ... Stoichiometry (sometimes called reaction stoichiometry to distinguish it from composition stoichiometry) is the calculation of quantitative (measurable) relationships of the reactants and products in chemical reactions (chemical equations). ... Mole Day is an unofficial holiday celebrated among chemists in North America on October 23, between 6:02 AM and 6:02 PM, making the date 6:02 10/23 in the American style of writing dates. ... For other uses, see Concentration (disambiguation). ... CODATA (Committee on Data for Science and Technology) was established in 1966 as an interdisciplinary committee of the International Council of Science (ICSU), formerly the International Council of Scientific Unions. ...

References