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Osmolarity is a measurement of the number of particles of solute, expressed in osmoles, in a liter of a solution. This measurement can be difficult to obtain because the liter of solution will be subject to temperature and pressure changes that may cause it to expand and contract. Preferentially, some scientists prefer to measure osmolality, which looks at the number of osmoles in a kilogram of material. The weight will remain relatively stable, assuming the scientist does not go into outer space, and thus the measurement will be more accurate.
Measurements of osmolarity include all of the solutes in a solution, including those that are capable of moving across a semipermeable membrane and those that are not. It is also an absolute measurement. These two features distinguish it from tonicity, a related measurement that may be useful in some settings. The two concepts are sometimes confused because they appear to be measures of the same thing, since they both deal with assessments of solutes in a solvent mixture.
When a researcher measures tonicity, he is interested in the relationship between two fluid solutions. If the solutions are isotonic, they have the same tonicity. If one is hypotonic to the other, it has a lower concentration of solutes. The tonicity of two solutions determines what moves across a semipermeable membrane, and when. Osmosis plays an important role in the lives and function of many organisms and can become a liability if cells are bathed in a solution that is too hypotonic or hypertonic, as the cells may swell so much they explode, or lose so much water that they die.
Knowing the osmolarity of a solution will provide information about its concentration, but this does not necessarily provide any data about its tonicity. The measurement doesn't distinguish between different solutes and thus a mixture might have an osmolarity of three, but that doesn't tell a researcher which of the solutes, if any, are capable of passing through a semipermeable membrane. Measurements of osmolarity can be useful for activities like testing samples from a patient to check on the levels of dissolved salts and other compounds.
Blood and urine sample reports often discuss the osmolarity of the total solution. The test can also offer a breakdown of the different compounds found and their levels. This information can help a doctor determine the nature of a patient's medical problem. It can also help in an assessment to see if a patient is responding to medications.