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The calculated osmolality is then subtracted from the measured osmolality to provide the osmol gap, or the difference between these two values. If this gap falls within an acceptable range,(<10) then it is assumed that sodium, glucose, BUN are indeed the major dissolved ions and molecules in the serum.
Note: NaCl does not dissolve completely in water at standard temperature and pressure, so the solution will be comprised of Na+ ions, Cl- ions, and some NaCl molecules, with actual osmolality = Na+ concentration x 1.75] Another example is magnesium chloride (MgCl 2), which dissociates into Mg 2+ and 2Cl − ions.
The laboratory value measures the freezing point depression, properly called osmolality while the calculated value is given in units of osmolarity. Even though these values are presented in different units, when there is a small amount of solute compared to total volume of solution, the absolute values of osmolality vs. osmolarity are very close.
Osmosis in a U-shaped tube. Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. [1]
For most ionic compounds dissolved in water, the van 't Hoff factor is equal to the number of discrete ions in a formula unit of the substance. This is true for ideal solutions only, as occasionally ion pairing occurs in solution. At a given instant a small percentage of the ions are paired and count as a single particle.
Stool osmotic gap is a measurement of the difference in solute types between serum and feces, used to distinguish among different causes of diarrhea. Feces is normally in osmotic equilibrium with blood serum, which the human body maintains between 290–300 mOsm/kg. [1] However, the solutes contributing to this total differ.
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Moreover, specific surface area is a scale-dependent quantity, and no single true value exists for this parameter. [1] Thus, the use of alternative probe molecules can often result in different obtained numerical values for surface area, rendering comparison problematic. The model also ignores adsorbate–adsorbate interactions.