Wikipedia / Molar concentration
Wikipedia / Molar concentration
Molar concentration (also called amount-of-substance concentration or molarity) is the number of moles of solute per liter of solution. Specifically, it is a measure of the concentration of a chemical species, in particular, of a solute in a solution, in terms of amount of substance per unit volume of solution. In chemistry, the most commonly used unit for molarity is the number of moles per liter, having the unit symbol mol/L or mol/dm3 (1000mol/m3) (really (mol/m3)/1000) in SI units. Molar concentration is often depicted with square brackets around the substance of interest; for example, the molarity of the hydronium ion is denoted as [H3O+].
Definition
Molar concentration, or molarity, is most commonly expressed in units of moles of solute per litre of solution. For use in broader applications, it is defined as amount of substance:
is the amount of the solute in moles, is the number of constituent particles is the Avogadro constant, since 2019 defined as exactly is the number density.
In thermodynamics, the use of molar concentration is often not convenient because the volume of most solutions slightly depends on temperature due to thermal expansion. This problem is usually resolved by introducing temperature correction factors, or by using a temperature-independent measure of concentration such as molality.
The reciprocal quantity represents the dilution (volume) which can appear in Ostwald's law of dilution.
Formality or analytical concentration
If a molecule or salt dissociates in solution, the concentration refers to the original chemical formula in solution, the molar concentration is sometimes called formal concentration or formality (FA) or analytical concentration (cA). For example, if a sodium carbonate solution () has a formal concentration of c() = 1mol/L, the molar concentrations are c() = 2mol/L and c() = 1mol/L because the salt dissociates into these ions.
Units
While there is clear consensus on the equivalence of units:
guidance on unit names and abbreviations varies:
The SI prefix "mega" (symbol M) has the same symbol. However, the prefix is never used alone, so "M" unambiguously denotes molar. Sub-multiples, such as "millimolar" (mM) and "nanomolar" (nM), consist of the unit preceded by an SI prefix:
Related quantities
Number concentration
The conversion to number concentration is given by
is the Avogadro constant.
Mass concentration
The conversion to mass concentration is given by
is the molar mass.
Mole fraction
The conversion to mole fraction is given by
is the density of the solution.
A simpler relation can be obtained by considering the total molar concentration, namely, the sum of molar concentrations of all the components of the mixture:
Mass fraction
The conversion to mass fraction is given by
Molality
For binary mixtures, the conversion to molality is
where the solvent is substance 1, and the solute is substance 2.
For solutions with more than one solute, the conversion is
Properties
Sum of molar concentrations – normalizing relations
The sum of molar concentrations gives the total molar concentration, namely the density of the mixture divided by the molar mass of the mixture or by another name the reciprocal of the molar volume of the mixture. In an ionic solution, ionic strength is proportional to the sum of the molar concentration of salts.
Sum of products of molar concentrations and partial molar volumes
The sum of products between these quantities equals one:
Dependence on volume
The molar concentration depends on the variation of the volume of the solution due mainly to thermal expansion. On small intervals of temperature, the dependence is
is the thermal expansion coefficient of the mixture.
Examples
See also
References
External links