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. Chemistry (from Greek χημεία khemeia[1] meaning alchemy) is the science of matter at the atomic to molecular scale, dealing primarily with collections of atoms, such as molecules, crystals, and metals. ... Cover of brochure The International System of Units. ... The mole (symbol: mol) is the SI base unit that measures an amount of substance. ... A chemical compound is a chemical substance consisting of two or more different chemically bonded chemical elements, with a fixed ratio determining the composition. ... Dissolving table salt in water In chemistry, a solution is a homogeneous mixture composed of one or more substances, known as solutes, dissolved in another substance, known as a solvent. ... Osmotic pressure or turgor (also called turgor pressure) is the pressure produced by a solution in a space that is enclosed by a differentially permeable membrane. ...

Osmolarity is a measure of the osmoles of solute per litre 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 litre or liter (U.S. spelling, see spelling differences) is a unit of volume. ... The international prototype, made of platinum-iridium, which is kept at the BIPM under conditions specified by the 1st CGPM in 1889. ... A solvent is a fluid phase (liquid, gas, or plasma) that dissolves a solid, liquid, or gaseous solute, resulting in a solution. ... In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. ... A magnified crystal of a salt (halite/sodium chloride) A salt, in chemistry, is any ionic compound composed of cations (positively charged ions) and anions (negative ions) so that the product is neutral (without a net charge). ... An ion is an atom or group of atoms that normally are electrically neutral and achieve their status as an ion by loss or addition of one or more electrons. ... Glucose (Glc), a monosaccharide (or simple sugar), is the most important carbohydrate in biology. ... Sodium chloride, also known as common salt, table salt, or halite, is a chemical compound with the formula NaCl. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... 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 are also called chlorides. ...

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

where

• Φ is the osmotic coefficient and accounts for the degree of dissociation of the solute. Φ is between 0 and 1 where 1 indicates 100% dissociation.
• 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 uses the principle of Freezing-point depression to determine the osmolality. Freezing-point depression is the difference between the freezing points of a pure solvent and a solution mixed with a solute. ...

See also

• Molarity
• Molality
• Plasma osmolality
• Tonicity

This page refers to concentration in the chemical sense. ... This page refers to concentration in the chemical sense. ... Plasma osmolality is the osmolality of human blood. ... Tonicity is the ability of a solution to cause water movement. ...

External link

• MedlinePlus Medical Encyclopedia