Molar concentration
Molar concentration | |
---|---|
Common symbols | c, [chemical symbol or formula] |
SI unit | mol/m^{3} |
Other units | mol/L |
Derivations from other quantities | c = n/V |
Dimension |
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 units. A solution with a concentration of 1 mol/L is said to be 1 molar, commonly designated as 1 M or 1 M. Molarity is often depicted with square brackets around the substance of interest; for example, the molarity of the hydrogen ion is depicted as [H^{+}].
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 :
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 2019 defined as exactly 6.02214076×10^{23} 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.
The reciprocal quantity represents the dilution (volume) 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 (F_{A}) or analytical concentration (c_{A}). For example, if a sodium carbonate solution (Na_{2}CO_{3}) has a formal concentration of c(Na_{2}CO_{3}) = 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.
Units
In the International System of Units (SI), the coherent unit for molar concentration is mol/m^{3}. However, most chemical literature traditionally uses mol/dm^{3}, which is the same as mol/L. 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:
Name | Abbreviation | Concentration | |
---|---|---|---|
(mol/L) | (mol/m^{3}) | ||
millimolar | mM | 10^{−3} | 10^{0}=1 |
micromolar | μM | 10^{−6} | 10^{−3} |
nanomolar | nM | 10^{−9} | 10^{−6} |
picomolar | pM | 10^{−12} | 10^{−9} |
femtomolar | fM | 10^{−15} | 10^{−12} |
attomolar | aM | 10^{−18} | 10^{−15} |
zeptomolar | zM | 10^{−21} | 10^{−18} |
yoctomolar | yM | 10^{−24} (6 particles per 10 L) |
10^{−21} |
rontomolar | rM | 10^{−27} | 10^{−24} |
quectomolar | qM | 10^{−30} | 10^{−27} |
Related quantities
Number concentration
The conversion to number concentration is given by
where is the Avogadro constant.
Mass concentration
The conversion to mass concentration is given by
where is the molar mass of constituent .
Mole fraction
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:
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
where is the molar concentration at a reference temperature, is the thermal expansion coefficient of the mixture.
Examples
- 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 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
- 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(H_{2}O) = 1000 g/L/18.02 g/mol ≈ 55.5 mol/L.
- c(H_{2}) = 88 g/L/2.02 g/mol = 43.7 mol/L.
- c(OsO_{4}) = 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) = 6×10^{16} L^{−1}.
The molar concentration is- c = C/N_{A} = 6×10^{16} L^{−1}/6×10^{23} mol^{−1} = 10^{−7} mol/L = 100 nmol/L.
- Reference ranges for blood tests, sorted by molar concentration: