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. In chemistry, the most commonly used unit for molarity is the number of moles per liter, having the unit symbol mol/L or mol⋅dm−3 in SI unit. A solution with a concentration of 1 mol/L is said to be 1 molar, commonly designated as 1 M. To avoid confusion with SI prefix mega, which has the same abbreviation, small caps ᴍ or italicized M are also used in journals and textbooks.
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 :
Here, is the amount of the solute in moles, is the number of constituent particles present in volume (in litres) of the solution, and is the Avogadro constant, since 24 January 2021 defined as exactly 6.02214076×1023 mol−1. The ratio 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.
- 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 (FA) or analytical concentration (cA). 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.
In the International System of Units (SI) 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:
(6 particles per 10 L)
The conversion to number concentration is given by
where is the Avogadro constant.
The conversion to mass concentration is given by
where is the molar mass of constituent .
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 mass fraction is given by
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
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
where is the molar concentration at a reference temperature, is the thermal expansion coefficient of the mixture.
- 11.6 g of NaCl is dissolved in 100 g of water. The final mass concentration ρ(NaCl) is
- ρ(NaCl) = 11.6 g/11.6 g + 100 g = 0.104 g/g = 10.4 %.
The density of such a solution is 1.07 g/mL, thus its volume is
- V = 11.6 g + 100 g/1.07 g/mL = 104.3 mL.
The molar concentration of NaCl in the solution is therefore
- c(NaCl) = 11.6 g/58 g/mol / 104.3 mL = 0.00192 mol/mL = 1.92 mol/L.
- A typical task in chemistry is the preparation of 100 mL (= 0.1 L) of a 2 mol/L solution of NaCl in water. The mass of salt needed is
- m(NaCl) = 2 mol/L × 0.1 L × 58 g/mol = 11.6 g.
- The density of water is approximately 1000 g/L and its molar mass is 18.02 g/mol (or 1/18.02 = 0.055 mol/g). Therefore, the molar concentration of water is
- c(H2O) = 1000 g/L/18.02 g/mol ≈ 55.5 mol/L.
- c(H2) = 88 g/L/2.02 g/mol = 43.7 mol/L.
- c(OsO4) = 5.1 kg/L/254.23 g/mol = 20.1 mol/L.
- A typical protein in bacteria, such as E. coli, may have about 60 copies, and the volume of a bacterium is about 10−15 L. Thus, the number concentration C is
- C = 60 / (10−15 L) = 6×1016 L−1.The molar concentration is
- c = C/NA = 6×1016 L−1/6×1023 mol−1 = 10−7 mol/L = 100 nmol/L.
- Reference ranges for blood tests, sorted by molar concentration:
- "Typography of unit symbols for Molar and Liter in siunitx". TeX - LaTeX Stack Exchange.
- Tro, Nivaldo J. Introductory chemistry essentials (Fifth ed.). Boston. p. 457. ISBN 9780321919052. OCLC 857356651.
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "amount concentration, c". doi:10.1351/goldbook.A00295
- Kaufman, Myron (2002). Principles of thermodynamics. CRC Press. p. 213. ISBN 0-8247-0692-7.
- David Bradley. "How low can you go? The Y to Y".