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 of solute per unit volume of solution, or per unit volume available to the species, represented by lowercase c: Here, n is the amount of the solute in moles, N is the number of constituent particles present in volume V of the solution, and NA is the Avogadro constant, since 20 May 2019 defined as exactly. The ratio N/V is the number densityC. 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 quantity represents the dilution which can appear in Ostwald's law of dilution. ; Formality or analytical concentration If a molecular entity dissociates in solution, the concentration refers to the original chemical formula in solution, the molar concentration is sometimes called formal concentration or formality or analytical concentration. For example, if a sodium carbonate solution has a formal concentration of c = 1 mol/L, the molar concentrations are c = 2 mol/L and c = 1 mol/L because the salt dissociates into these ions.
Units
In the International System of Units the coherent unit for molar concentration is mol/m3. However, this is inconvenient for most laboratory purposes and most chemical literature traditionally uses mol/dm3, which is the same as mol/L. This traditional unit is often denoted by the letter M, optionally preceded by an SI prefix as needed to denote sub-multiples, for example: The units millimolar and micromolarrefer to mM and μM, respectively.
Related quantities
Number concentration
The conversion to number concentration is given by where is the Avogadro constant.
The conversion to mole fraction is given by where is the average molar mass of the solution, 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:
The conversion to molality is where the solute is assigned the subscript 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 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 where is the molar concentration at a reference temperature, is the thermal expansion coefficient of the mixture.
