Amount of substance

Physical size
Surname	Amount of substance
Formula symbol	${\displaystyle n}$
Size and unit system unit dimension SI mol N

The amount of substance (outdated molar amount or molar number ) with the formula symbol is a basic quantity in the International System of Units  (SI) and indirectly indicates the number of particles in a substance portion. Particles can be atoms , ions , molecules , formula units or electrons . Formula symbol and particle type X are given together as n _X or n (X). ${\displaystyle n}$

The unit of measurement for the amount of substance is the mole , an SI base unit. An amount of substance of 1 mol ( = 1 mol) contains that determined by the Avogadro constant ( N _A ≈ 6 ${\displaystyle n}$ .022e²³ mol ⁻¹ ) specified number of particles. The Avogadro constant is theproportionality factorbetween the amount of substance and the number of particles N(X):

${\displaystyle {\begin{aligned}n(X)\cdot N_{\mathrm {A} }&=N(X)\\\Leftrightarrow n(X)&={\frac {N(X)}{N_{\mathrm {A} }}}\end{aligned}}}$

Amount of substance and the quantities derived from it, such as concentration of substance , amount of substance and ratio of substance, are important in stoichiometry . The use of the quantity of substance shifts considerations of chemical reactions from the atomic or molecular range to weighable substance masses with a very high number of particles.

For the amount of substance  n _X and the mass  m _{X of} a substance portion of a substance X and its molar mass  M _X :

${\displaystyle n_{\mathrm {X} }={m_{\mathrm {X} } \over M_{\mathrm {X} }}}$

Calculating the amount of substance

The relationship between mass, amount of substance, volume and number of particles.

The amount of substance is calculated

... from the crowd

The calculation from the mass is possible using the equation given above.

Example: The molar mass of water is 18 grams per mole. 9 grams of water correspond to an amount of substance of 0.5 mol.

${\displaystyle n_{\mathrm {H_{2}O} }={\frac {m_{\mathrm {H_{2}O} }}{M_{\mathrm {H_{2}O} }}}=\mathrm {9\;g \over {18\;{g \over mol}}} =0{,}5\,\mathrm {mol} }$

... from the volume

One mole of an ideal gas takes up an approximate volume of V _m  = 22.4 l / mol under standard conditions . This volume is called the standard molar volume (molar volume). The amount of substance results directly from the volume.

Example: How many moles is equivalent to 5 liters of oxygen ?

${\displaystyle n_{\mathrm {O_{2}} }={V_{\mathrm {O_{2}} } \over {V_{\mathrm {m} }}}={{5\,\mathrm {l} } \over {22{,}4{\mathrm {l \over mol} }}}=0{,}22\,\mathrm {mol} }$

... from the number of particles

Since the number of particles N is proportional to the amount of substance n (the proportionality factor is the Avogadro constant N _A = 6.022 · 10 ²³ / mol), the amount of substance can be calculated from the number of particles.

Example: Given are N = 10 ²⁵ particles.

${\displaystyle n={\frac {N}{N_{\mathrm {A} }}}={\frac {10^{25}}{6{,}022\cdot 10^{23}\;\mathrm {mol} ^{-1}}}=16{,}606\;\mathrm {mol} }$

... out of concentration

Since the concentration c _X (mol / l) is a measure of the concentration for solutions , which relates the amount of substance X to the volume V of the solution, this can also be calculated back to the amount of substance.

Example: How many moles of sodium chloride are in 0.22 liters of a 0.6 molar NaCl solution?

${\displaystyle n_{\mathrm {NaCl} }=c_{\mathrm {NaCl} }\cdot V=0{,}6\;{\frac {\mathrm {mol} }{\mathrm {l} }}\cdot 0{,}22\;\mathrm {l} =0{,}132\;\mathrm {mol} }$

See also

• Salary statement
• Order of magnitude (amount of substance)

Web links

Wiktionary: Amount of substance  - explanations of meanings, word origins, synonyms, translations

Individual evidence

1. ↑ Entry on amount of substance . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.A00297 Version: 2.3.2.