## UNIT: mole

• PID: si:unit:mole
• Definition: The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 × 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol-1 and is called the Avogadro number. The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.
• Source: SI Brochure 9th Ed. 2019, p 134
• Reference: CGPM Resolution 1 of the 26th CGPM (2018) "On the revision of the International System of Units (SI)"
• Status: Valid
• Valid: 2019-05-20 -
• Notes
1. This definition implies the exact relation $${\rm{N_{A}}} = 6.022\:140\:76 \times 10^{23} {\rm{mol}}^{-1}$$. Inverting this relation gives an exact expression for the mole in terms of the defining constant $$\rm{N_{A}}$$: $$1\ {\rm{mol}} = \left( \frac{6.022\:140\,76 \times 10^{23}}{N_{\rm{A}} }\right).$$
2. The effect of this definition is that the mole is the amount of substance of a system that contains $$6.022\:140\:76 \times 10^{23}$$ specified elementary entities.
3. The previous definition of the mole fixed the value of the molar mass of carbon 12, $$M(^{12}\rm{C})$$, to be exactly $$0.012\ {\rm{kg/mol}}$$. According to the present definition $$M(^{12}\rm{C})$$ is no longer known exactly and must be determined experimentally. The value chosen for $$\rm{N_{A}}$$ is such that at the time of adopting the present definition of the mole, $$M(^{12}\rm{C})$$ was equal to $$0.012\ {\rm{kg/mol}}$$ with a relative standard uncertainty of $$4.5 \times 10^{-10}$$.
4. The molar mass of any atom or molecule X may still be obtained from its relative atomic mass from the equation $$M(X)=A_{\rm{r}}(X)[M(^{12}{\rm{C}})/12]=A_{\rm{r}}(X)M_{\rm{u}}$$ and the molar mass of any atom or molecule X is also related to the mass of the elementary entity $$m(X)$$ by the relation $$M(X)=N_{\rm{A}}m(X)=N_{\rm{A}}A_{\rm{r}}(X)m_{\rm{u}}.$$
5. In these equations $$M_{\rm{u}}$$ is the molar mass constant, equal to $$M(^{12}{\rm{C}})/12$$ and $$m_{\rm{u}}$$ is the unified atomic mass constant, equal to $$m(^{12}{\rm{C}})/12$$. They are related to the Avogadro constant through the relation $$M_{\rm{u}}=N_{\rm{A}}m_{\rm{u}}$$
6. In the name "amount of substance", the word "substance" will typically be replaced by words to specify the substance concerned in any particular application, for example "amount of hydrogen chloride", or "amount of benzene". It is important to give a precise definition of the entity involved (as emphasized in the definition of the mole); this should preferably be done by specifying the molecular chemical formula of the material involved. Although the word "amount" has a more general dictionary definition, the abbreviation of the full name "amount of substance" to "amount" may be used for brevity. This also applies to derived quantities such as "amount-of-substance concentration", which may simply be called "amount concentration". In the field of clinical chemistry, the name "amount-of-substance concentration" is generally abbreviated to "substance concentration".
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