Concentration#Molality

Concentration-, concentratio, action or an act of coming together at a single place, bringing to a common center, was used in post-classical Latin in 1550 or earlier, similar terms attested in Italian (1589), Spanish (1589), English (1606), French (1632).{{Cite OED|concentration|id=38114}}

Image:Dilution-concentration simple example.jpg

Often in informal, non-technical language, concentration is described in a qualitative way, through the use of adjectives such as "dilute" for solutions of relatively low concentration and "concentrated" for solutions of relatively high concentration. To concentrate a solution, one must add more solute (for example, alcohol), or reduce the amount of solvent (for example, water). By contrast, to dilute a solution, one must add more solvent, or reduce the amount of solute. Unless two substances are miscible, there exists a concentration at which no further solute will dissolve in a solution. At this point, the solution is said to be saturated. If additional solute is added to a saturated solution, it will not dissolve, except in certain circumstances, when supersaturation may occur. Instead, phase separation will occur, leading to coexisting phases, either completely separated or mixed as a suspension. The point of saturation depends on many variables, such as ambient temperature and the precise chemical nature of the solvent and solute.

Concentrations are often called levels, reflecting the mental schema of levels on the vertical axis of a graph, which can be high or low (for example, "high serum levels of bilirubin" are concentrations of bilirubin in the blood serum that are greater than normal).

There are four quantities that describe concentration:

{{main|Mass concentration (chemistry)}}

The mass concentration $\backslash rho\_i$ is defined as the mass of a constituent $m\_i$ divided by the volume of the mixture $V$:

:$\backslash rho\_i\; =\; \backslash frac\; \{m\_i\}\{V\}.$

The SI unit is kg/m³ (equal to g/L).

{{main|Molar concentration}}

The molar concentration $c\_i$ is defined as the amount of a constituent $n\_i$ (in moles) divided by the volume of the mixture $V$:

:$c\_i\; =\; \backslash frac\; \{n\_i\}\{V\}.$

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

{{main|Number concentration}}

The number concentration $C\_i$ is defined as the number of entities of a constituent $N\_i$ in a mixture divided by the volume of the mixture $V$:

:$C\_i\; =\; \backslash frac\{N\_i\}\{V\}.$

The SI unit is 1/m³.

The volume concentration $\backslash sigma\_i$ (not to be confused with volume fraction{{GoldBookRef | file = V06643 | title = volume fraction}}) is defined as the volume of a constituent $V\_i$ divided by the volume of the mixture $V$:

:$\backslash sigma\_i\; =\; \backslash frac\; \{V\_i\}\{V\}.$

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

There seems to be no standard notation in the English literature. The letter $\backslash sigma\_i$ used here is normative in German literature (see Volumenkonzentration).

Several other quantities can be used to describe the composition of a mixture. These should not be called concentrations.

{{main|Normality (chemistry)}}

Normality is defined as the molar concentration $c\_i$ divided by an equivalence factor $f\_\backslash mathrm\{eq\}$. Since the definition of the equivalence factor depends on context (which reaction is being studied), the International Union of Pure and Applied Chemistry and National Institute of Standards and Technology discourage the use of normality.

{{main|Molality}}{{Distinguish|Molarity}}

The molality of a solution $b\_i$ is defined as the amount of a constituent $n\_i$ (in moles) divided by the mass of the solvent $m\_\backslash mathrm\{solvent\}$ (not the mass of the solution):

:$b\_i\; =\; \backslash frac\{n\_i\}\{m\_\backslash mathrm\{solvent\}\}.$

The SI unit for molality is mol/kg.

{{main|Mole fraction}}

The mole fraction $x\_i$ is defined as the amount of a constituent $n\_i$ (in moles) divided by the total amount of all constituents in a mixture $n\_\backslash mathrm\{tot\}$:

:$x\_i\; =\; \backslash frac\; \{n\_i\}\{n\_\backslash mathrm\{tot\}\}.$

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

{{main|Mixing ratio}}

The mole ratio $r\_i$ is defined as the amount of a constituent $n\_i$ divided by the total amount of all other constituents in a mixture:

:$r\_i\; =\; \backslash frac\{n\_i\}\{n\_\backslash mathrm\{tot\}-n\_i\}.$

If $n\_i$ is much smaller than $n\_\backslash mathrm\{tot\}$, 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.

{{main|Mass fraction (chemistry)}}

The mass fraction $w\_i$ is the fraction of one substance with mass $m\_i$ to the mass of the total mixture $m\_\backslash mathrm\{tot\}$, defined as:

:$w\_i\; =\; \backslash frac\; \{m\_i\}\{m\_\backslash mathrm\{tot\}\}.$

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

{{main|Mixing ratio}}

The mass ratio $\backslash zeta\_i$ is defined as the mass of a constituent $m\_i$ divided by the total mass of all other constituents in a mixture:

:$\backslash zeta\_i\; =\; \backslash frac\{m\_i\}\{m\_\backslash mathrm\{tot\}-m\_i\}.$

If $m\_i$ is much smaller than $m\_\backslash mathrm\{tot\}$, 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.

Concentration depends on the variation of the volume of the solution with temperature, due mainly to thermal expansion.

• {{annotated link|Dilution ratio}}
• {{annotated link|Dose concentration}}
• {{annotated link|Serial dilution}}
• {{annotated link|Wine/water mixing problem}}
• {{section link|Standard state|Solutes}}

{{Reflist}}

• {{Commonscatinline|Concentration (chemistry)}}

{{Chemical solutions}}

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