In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Furthermore, in chemistry, four types of mathematical description can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration.[1] The term concentration can be applied to any kind of chemical mixture, but most frequently it refers to solutes in homogeneous solutions.
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Often in informal, non-technical language, concentration is described in a qualitative way, through the use of adjectives such as "dilute" for solutions of relatively low concentration and "concentrated" for solutions of relatively high concentration. To concentrate a solution, one must add more solute (for example, alcohol), or reduce the amount of solvent (for example, water). By contrast, to dilute a solution, one must add more solvent, or reduce the amount of solute. Unless two substances are fully miscible there exists a concentration at which no further solute will dissolve in a solution. At this point, the solution is said to be saturated. If additional solute is added to a saturated solution, it will not dissolve, except in certain circumstances, when supersaturation may occur. Instead, phase separation will occur, leading to coexisting phases, either completely separated or mixed as a suspension. The point of saturation depends on many variables such as ambient temperature and the precise chemical nature of the solvent and solute.
There are four quantities that describe concentration:
The mass concentration is defined as the mass of a constituent divided by the volume of the mixture :
The SI-unit is kg/m3.
The molar concentration is defined as the amount of a constituent divided by the volume of the mixture :
The SI-unit is mol/m3. However, more commonly the unit mol/L is used.
The number concentration is defined as the number of entities of a constituent in a mixture divided by the volume of the mixture :
The SI-unit is 1/m3.
The volume concentration (also called volume fraction) is defined as the volume of a constituent divided by the volume of all consituents of the mixture prior to mixing:
The SI-unit is m3/m3.
Several other quantities can be used to describe the composition of a mixture. Note that these should not be called concentrations.
Normality is defined as the molar concentration divided by an equivalence factor . Since the definition of the equivalence factor may not be unequivocal, IUPAC and NIST discourage the use of normality.
The molality of a solution is defined as the amount of a constituent divided by the mass of the solvent (not the mass of the solution):
The SI-unit for molality is mol/kg.
The mole fraction is defined as the amount of a constituent divided by the total amount of all constituents in a mixture :
The SI-unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole fractions.
The mole ratio is defined as the amount of a constituent divided by the total amount of all other constituents in a mixture:
If is much smaller than , the mole ratio is almost identical to the mole fraction.
The SI-unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole ratios.
The mass fraction is the fraction of one substance with mass to the mass of the total mixture , defined as:
The SI-unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass fractions.
The mass ratio is defined as the mass of a constituent divided by the total mass of all other constituents in a mixture:
If is much smaller than , the mass ratio is almost identical to the mass fraction.
The SI-unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass ratios.
Concentration depends on the variation of the volume of the solution due mainly to thermal expansion.
Concentration type | Symbol | Definition | SI-unit | other unit(s) |
---|---|---|---|---|
mass concentration | or | kg/m3 | g/100mL (=g/dL) | |
molar concentration | mol/m3 | M (=mol/L) | ||
number concentration | 1/m3 | 1/cm3 | ||
volume concentration | m3/m3 | |||
Related quantities | Symbol | Definition | SI-unit | other unit(s) |
normality | mol/m3 | M (=mol/L) | ||
molality | mol/kg | |||
mole fraction | mol/mol | ppm, ppb, ppt | ||
mole ratio | mol/mol | ppm, ppb, ppt | ||
mass fraction | kg/kg | ppm, ppb, ppt | ||
mass ratio | kg/kg | ppm, ppb, ppt |
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