Osmolality

If 180 grams of glucose and 180 grams of fructose were dissolved in the same kilogram of water, the osmotic pressure of the solution would be the same as that of a 360-g/L glucose solution. Osmotic pressure depends on the ratio of solute to solvent, not on the chemical nature of the solute molecules. The expression for the total molality of a solution is osmolality (Osm). Thus, the solution of 1.0 m glucose plus 1.0 m fructose has a total molality, or osmolality, of 2.0 osmol/L (abbreviated 2.0 Osm). This osmo-lality is the same as that of the 360-g/L glucose solution, which has a concentration of 2.0 m and 2.0 Osm (fig. 6.9).

Unlike glucose, fructose, and sucrose, electrolytes such as NaCl ionize when they dissolve in water. One molecule of NaCl dissolved in water yields two ions (Na+ and Cl-); 1 mole of NaCl ionizes to form 1 mole of Na+ and 1 mole of Cl-. Thus, a 1.0 m NaCl solution has a total concentration of 2.0 Osm. The effect of this ionization on osmosis is illustrated in figure 6.10.

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