Amount of substance
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2008-06-05T05:13:30Z
Ashishbhatnagar72
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The '''amount of substance''', ''n'', of a sample or system is a [[physical quantity]] which is [[Proportionality|proportional]] to the number of [[Elementary entity|elementary entities]] present. "Elementary entities" may be atoms, molecules, ions, electrons, or particles, the choice of which is dependent upon context and must be stated. Amount of substance is sometimes referred to as '''chemical amount''' or, incorrectly, as '''number of moles'''.<ref>To use "''number of moles''" to refer to amount of substance is no more correct than to use "number of seconds" to refer to time. The amount of substance is the same whatever unit is used to measure it.</ref>
The [[SI]] unit for amount of substance is the [[Mole (unit)|mole (mol)]], which is defined as the amount of substance that has an equal number of elementary entities as there are atoms in 12 [[gram|g]] of [[carbon]]-12. That number is the [[Avogadro constant]], ''N''<sub>A</sub>, which has a value<ref>{{CODATA|url=http://physics.nist.gov/cgi-bin/cuu/Value?na}}</ref> of {{val|6.02214179|(30)|e=23|u=mol{{su|p=−1}}}}.
==Rationale==
Why use amount of substance instead of [[mass]] or [[volume]] to tell how much of a substance there is? This is because in [[chemical reaction]]s, the reagents react [[molecule]]-to-molecule, ion-to-ion, etc. Since different atoms and therefore molecules have different masses, 100 grams of some substance is not same amount of some other substance. For example, 100 grams of [[carbon]] has more molecules than 100 grams of [[oxygen]].
See: [[Stoichiometry]]
== Terminology ==
{{wiktionary|amount}}
When quoting an amount of substance, it is necessary to specify the entity involved (unless there is no risk of ambiguity). One mole of [[chlorine]] could refer either to chlorine atoms (as in 58.44 g of [[sodium chloride]]) or to chlorine molecules (as in {{val|22.711|u=dm3}} of chlorine gas at [[STP]]). The simplest way to avoid ambiguity is to replace the term "substance" by the name of the entity and/or to quote the [[empirical formula]]. For example:
:amount of [[chloroform]], CHCl<sub>3</sub>
:amount of [[sodium]], Na
:amount of [[hydrogen]] (atoms), H
:''n''([[ethylene|C<sub>2</sub>H<sub>4</sub>]])
This can be considered to be a technical definition of the word "amount", a usage which is also found in the names of certain derived quantities (see below).
== Derived quantities ==
When amount of substance enters into a [[derived quantity]], it is usually as the denominator: such quantities are known as "[[Molar quantity|molar quantities]]".<ref name="molar">{{GreenBookRef|page=7}}</ref> For example, the quantity which describes the volume occupied by a given amount of substance is called the [[molar volume]], while the quantity which describes the mass of a given amount of substance is the [[molar mass]]. Molar quantities are sometimes denoted by a subscript Latin "m" in the symbol,<ref name="molar"/> e.g. ''C''<sub>''p'',m</sub>, molar [[heat capacity]] at constant pressure: the subscript may be omitted if there is no risk of ambiguity, as is often the case in pure [[chemistry]].
The main derived quantity in which amount of substance enters into the numerator is [[amount of substance concentration]], ''c''. This name is often abbreviated to "amount concentration",<ref>{{GoldBookRef|title=amount-of-substance concentration|url=http://goldbook.iupac.org/A00298.html}}</ref> except in [[clinical chemistry]] where "substance concentration" is the preferred term<ref>[[International Union of Pure and Applied Chemistry]] (1996). "[http://www.iupac.org/publications/pac/1996/pdf/6804x0957.pdf Glossary of Terms in Quantities and Units in Clinical Chemistry.]" [[Pure and Applied Chemistry|''Pure Appl. Chem.'']] '''68''':957–1000.</ref> (to avoid any possible ambiguity with [[mass concentration]]). The name "molar concentration" is incorrect,<ref>"''Molar concentration''" should refer to a concentration per mole, i.e. an [[amount fraction]]. The use of "[[molar (unit)|]]" as a unit, equal to 1 mol/dm<sup>3</sup>, symbol M, is frequent, but not (as of May 2007) completely condoned by IUPAC: See {{GreenBookRef|page=42 (n. 15)}}</ref> if commonly used.
== See also ==
*[[Amount fraction]], ''x''
== References ==
*{{SIbrochure|pages=114–15}}
*{{GoldBookRef|title=amount of substance, ''n''|url=http://goldbook.iupac.org/A00297.html}}
*{{GreenBookRef|page=46}}
<references/>
[[Category:Physical quantity]]
[[Category:Chemical properties]]
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[[es:Cantidad de sustancia]]
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[[fr:Quantité de matière]]
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[[ro:Cantitate de substanţă]]
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