Osmolality and osmolarity

The experimentally derived osmotic pressure is frequently expressed as the osmolality %m, which is the mass of solute which, when dissolved in 1 kg of water, will exert an osmotic pressure, n', equal to that exerted by 1 mole of an ideal unionised substance dissolved in 1 kg of water. The unit of osmolality is the osmole (abbreviated as osmol), which is the amount of substance that dissociates in solution to form one mole of osmotically active particles, thus 1 mole of glucose (not ionised) forms 1 osmole of solute, whereas 1 mole of NaCl forms 2 osmoles (1 mole of Na+ and 1 mole of

Cl). In practical terms, this means that a 1 molal solution of NaCl will have (approximately) twice the osmolality (osmotic pressure) as a 1 molal solution of glucose.

According to the definition, = n/n'. The value of n' may be obtained from equation (3.59) by noting that for an ideal unionised substance v = <p = 1, and since m is also unity, equation (3.59) becomes n = © M

V1 j 1000


EXAMPLE 3.3 Calculation of osmolality

A 0.90% w/w solution of sodium chloride (mol. wt. = 58.5) has an osmotic coefficient of 0.928. Calculate the osmolality of the solution.


Osmolality is given by equation (3.60) as = vm4>

0 0

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