# osmolarity

In chemistry, the osmole (Osm) is a non-SI unit of measurement that defines the number of moles of a chemical compound that contribute to a solution's osmotic pressure.

Osmolarity is a measure of the osmoles of solute per liter of solution, while the osmolality is a measure of the osmoles of solute per kilogram of solvent. Molarity and Osmolarity are not commonly used in osmometry because they are temperature dependent; that is, water changes its volume with temperature. However, if the concentration is very low, osmolarity and osmolality are considered equivalent. In calculations for these two measurements, salts are presumed to dissociate into their component ions. For example, a mole of glucose in solution is one osmole, whereas a mole of sodium chloride in solution is two osmoles (one mole of sodium and one mole of chloride). Both sodium and chloride ions affect the osmotic pressure of the solution.

The equation to determine the osmolality of a solution is given by

where
• Φ is the osmotic coefficient, which accounts for the degree of non-ideality of the solution. In the simplest case it is the degree of dissociation of the solute. Then, Φ is between 0 and 1 where 1 indicates 100% dissociation. However, Φ can also be larger than 1 (e.g. for sucrose). For salts, electrostatic effects cause Φ to be smaller than 1 even if 100% dissociation occurs (see Debye-HÃ¼ckel equation).
• n is the number of particles into which a molecule dissociates. For example: Glucose equals 1 and NaCl equals 2.
• C is the molal concentration of the solution
The units are Osm/kg

Osmolality can be measured using an osmometer which measures colligative properties, such as Freezing-point depression, Vapor pressure, or Boiling-point elevation.

## Osmolality/osmolarity vs. tonicity

While similar, osmolarity and tonicity are not the same. The key difference between the two is that tonicity implies a membrane that is impermeable to the solutes on either side of it. This is not a necessary condition in the case of osmolarity. Osmolarity is a measure of the osmotically active particles in a solution and in fact makes no explicit assertion with respect to the solute permeability of any involved membranes.

The derivatives of the term: isosmotic, hyperosmotic, and hypoosmotic, should not be confused with isotonic, hypertonic and hypotonic.

Example: A urea solution that is isosmotic with respect to the cytosol of an erythrocyte is nonetheless not isotonic respective to the same erythrocyte. Urea freely diffuses across cellular membranes and is also an osmotically active particle. Normally, urea is present in a lower concentration in the nju of an erythrocyte than in an urea solution. Because urea is freely permeable to cell membranes and the concentration of urea is normally lower in the erythrocytes than in a urea solution, urea will diffuse down its concentration gradient into an erythrocyte placed into a urea solution. However, because urea is osmotically active, urea increases the solute concentration in the erythrocyte, which will then induce the osmosis of water into the cell. This can ultimately result in cell lysis. In retrospect, the isosmotic urea solution was in fact hypotonic with respect to the blood cell. Interestingly, even if the urea solution is hypoosmotic to the erythrocyte, urea will still diffuse into the cell along its concentration gradient.

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Si, si, or SI may refer to (all SI unless otherwise stated):

In language:
• One of two Italian words:
• sÃ¬ (accented) for "yes"
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The mole (symbol: mol) is the SI base unit that measures an amount of substance. One mole contains Avogadro's number (approximately 6.0221023) entities.

A mole is much like "a dozen" in that both are absolute numbers (having no units) and can describe any type of
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In chemistry, a solution is a homogeneous mixture composed of two or more substances.
Osmotic pressure is the hydrostatic pressure produced by a solution in a space divided by a semipermeable membrane due to a differential in the concentrations of solute.

Osmotic potential
The litre or liter (see spelling differences) is a unit of volume. There are two official symbols, namely the Latin letter L both in lower and upper case: l and L.
kilogram or kilogramme (symbol: kg) is the SI base unit of mass. The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK), which is almost exactly equal to the mass of one liter of water.
A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. The most common solvent in everyday life is water. Most other commonly-used solvents are organic (carbon-containing) chemicals. These are called organic solvents.
In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of
Salt is a mineral essential for animal life, composed primarily of sodium chloride. Salt for human consumption is produced in different forms: unrefined salt (such as sea salt), refined salt (table salt), and iodized salt.
ion is an atom or molecule which has lost or gained one or more electrons, making it positively or negatively charged. A negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion
Glucose (Glc), a monosaccharide (or simple sugar), is an important carbohydrate in biology. The living cell uses it as a source of energy and metabolic intermediate.
For sodium in the diet, see salt.

Sodium chloride, also known as common salt, table salt, or halite, is a chemical compound with the formula NaCl.
Sodium (IPA: /ˈsəʊdiəm/) is a chemical element which has the symbol Na (Latin: natrium), atomic number 11, atomic mass 22.9898 g/mol, common oxidation number +1.
The chloride ion is formed when the element chlorine picks up one electron to form an anion (negatively-charged ion) Cl . The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides.
The Debye-HÃ¼ckel limiting law, named for its developers Peter Debye and Erich HÃ¼ckel, provides one way to obtain activity coefficients [1]. Activities, rather than concentrations, are needed in many chemical calculations because solutions that contain ionic
An osmometer is a device for measuring the osmotic strength of a solution, colloid, or compound.

There are several different techniques employed in osmometry:

Vapor pressure osmometers
Colligative properties are properties of solutions that depend on the number of particles in a given volume of solvent and not on the mass of the particles. Colligative properties include: lowering of vapour pressure; elevation of boiling point; depression of freezing point;
Freezing-point depression decribes the phenomenon that the freezing point of a liquid (a solvent) is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent.
Vapor pressure, also known as vapour pressure, is the pressure of a vapor in equilibrium with its non-vapor phases. All liquids and solids have a tendency to evaporate to a gaseous form, and all gases have a tendency to condense back into their orignal form (either liquid
Boiling-point elevation decribes the phenomenon that the boiling point of a liquid (a solvent) will be higher when another compound is added, meaning that a solution has a higher boiling point than a pure solvent.
In chemistry, the osmole (Osm) is a non-SI unit of measurement that defines the number of moles of a chemical compound that contribute to a solution's osmotic pressure.
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In chemistry, the osmole (Osm) is a non-SI unit of measurement that defines the number of moles of a chemical compound that contribute to a solution's osmotic pressure.