Concentration

The mass concentration ${\displaystyle \rho _{i}}$ is defined as the mass of a constituent ${\displaystyle m_{i}}$ divided by the volume of the mixture ${\displaystyle V}$:

${\displaystyle \rho _{i}={\frac {m_{i}}{V}}.}$

The SI unit is kg/m³ (equal to g/L).

The molar concentration ${\displaystyle c_{i}}$ is defined as the amount of a constituent ${\displaystyle n_{i}}$ (in moles) divided by the volume of the mixture ${\displaystyle V}$:

${\displaystyle c_{i}={\frac {n_{i}}{V}}.}$

The SI unit is mol/m³. However, more commonly the unit mol/L (= mol/dm³) is used.

The number concentration ${\displaystyle C_{i}}$ is defined as the number of entities of a constituent ${\displaystyle N_{i}}$ in a mixture divided by the volume of the mixture ${\displaystyle V}$:

${\displaystyle C_{i}={\frac {N_{i}}{V}}.}$

The SI unit is 1/m³.

The volume concentration ${\displaystyle \sigma _{i}}$ (not to be confused with volume fraction[2]) is defined as the volume of a constituent ${\displaystyle V_{i}}$ divided by the volume of the mixture ${\displaystyle V}$:

${\displaystyle \sigma _{i}={\frac {V_{i}}{V}}.}$

Being dimensionless, it is expressed as a number, e.g., 0.18 or 18%; its unit is 1.

There seems to be no standard notation in the English literature. The letter ${\displaystyle \sigma _{i}}$ used here is normative in German literature (see Volumenkonzentration).

Normality is defined as the molar concentration ${\displaystyle c_{i}}$ divided by an equivalence factor ${\displaystyle f_{\mathrm {eq} }}$. Since the definition of the equivalence factor depends on context (which reaction is being studied), IUPAC and NIST discourage the use of normality.

(Not to be confused with Molarity)

The molality of a solution ${\displaystyle b_{i}}$ is defined as the amount of a constituent ${\displaystyle n_{i}}$ (in moles) divided by the mass of the solvent ${\displaystyle m_{\mathrm {solvent} }}$ (not the mass of the solution):

${\displaystyle b_{i}={\frac {n_{i}}{m_{\mathrm {solvent} }}}.}$

The SI unit for molality is mol/kg.

The mole fraction ${\displaystyle x_{i}}$ is defined as the amount of a constituent ${\displaystyle n_{i}}$ (in moles) divided by the total amount of all constituents in a mixture ${\displaystyle n_{\mathrm {tot} }}$:

${\displaystyle x_{i}={\frac {n_{i}}{n_{\mathrm {tot} }}}.}$

The SI unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole fractions.

The mole ratio ${\displaystyle r_{i}}$ is defined as the amount of a constituent ${\displaystyle n_{i}}$ divided by the total amount of all other constituents in a mixture:

${\displaystyle r_{i}={\frac {n_{i}}{n_{\mathrm {tot} }-n_{i}}}.}$

If ${\displaystyle n_{i}}$ is much smaller than ${\displaystyle n_{\mathrm {tot} }}$, the mole ratio is almost identical to the mole fraction.

The SI unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole ratios.

The mass fraction ${\displaystyle w_{i}}$ is the fraction of one substance with mass ${\displaystyle m_{i}}$ to the mass of the total mixture ${\displaystyle m_{\mathrm {tot} }}$, defined as:

${\displaystyle w_{i}={\frac {m_{i}}{m_{\mathrm {tot} }}}.}$

The SI unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass fractions.

The mass ratio ${\displaystyle \zeta _{i}}$ is defined as the mass of a constituent ${\displaystyle m_{i}}$ divided by the total mass of all other constituents in a mixture:

${\displaystyle \zeta _{i}={\frac {m_{i}}{m_{\mathrm {tot} }-m_{i}}}.}$

If ${\displaystyle m_{i}}$ is much smaller than ${\displaystyle m_{\mathrm {tot} }}$, the mass ratio is almost identical to the mass fraction.

The SI unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass ratios.