Osmotic Pressure

Pressure which must be applied to a solution to prevent water from flowing in via a semipermeable membrane.

Osmotic pressure depends on the total number of dissolved particles, so for a substance that dissociates into two ions, such as ordinary salt (sodium chloride), a one molar solution has an osmotic pressure of 2 osmoles.

Solutions with higher concentrations of dissolved substances are said to have higher osmotic pressure than solutions having lower concentrations; thus the solvent moves from an area of low osmotic pressure to an area of higher osmotic pressure.

Osmotic Pressure
Fresh Water 20mOsm
Human Blood Plasma 290mOsm
Sea Water 1010mOsm

Values are typically small and so quoted in milliosmoles (mOsm)

See also: Hypertonic, Osmole, Osmosis, Reverse Osmosis.

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Subjects: Chemistry