Molar concentration (also called molarity, amount concentration or substance concentration) 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.

Molar concentration or molarity is most commonly expressed in units of moles of solute per litre of solution.

[1] 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

is the number of constituent particles present in volume

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.

For example, if a sodium carbonate solution (Na2CO3) has a formal concentration of c(Na2CO3) = 1 mol/L, the molar concentrations are c(Na+) = 2 mol/L and c(CO2−3) = 1 mol/L because the salt dissociates into these ions.

This traditional unit is often called a molar and denoted by the letter M, for example: 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: The conversion to number concentration

is the average molar mass 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 mass fraction

The sum of products between these quantities equals one: The molar concentration depends on the variation of the volume of the solution due mainly to thermal expansion.

is the molar concentration at a reference temperature,

is the thermal expansion coefficient of the mixture.

The volume of such a solution is 104.3mL (volume is directly observable); its density is calculated to be 1.07 (111.6g/104.3mL) The molar concentration of NaCl in the solution is therefore