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 (1000 mol/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 of solute per unit volume of solution, or per unit volume available to the species, represented by lowercase

c

{\displaystyle c}

:

c

=

n

V

=

N

N

A

V

=

C

N

A

.

{\displaystyle c={\frac {n}{V}}={\frac {N}{N_{\text{A}}\,V}}={\frac {C}{N_{\text{A}}}}.}

Here,

n

{\displaystyle n}

is the amount of the solute in moles,

N

{\displaystyle N}

is the number of constituent particles present in volume

V

{\displaystyle V}

(in litres) of the solution, and

N

A

{\displaystyle N_{\text{A}}}

is the Avogadro constant, since 2019 defined as exactly 6.02214076×1023 mol−1. The ratio

N

/

V

{\displaystyle {N}/{V}}

is the number density

C

{\displaystyle C}

.

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.