|
|
Units of Measure in Solutions:Concentrations are often given in terms of weight/volume. For example, mg/L, or mg/100 mL (common clinical units), are used. These units do not depend on knowledge of the molecular structure of the measured substance. For a substance with a known molecular structure, one can define a "Mole" of that substance. 1 Mole is the weight of 6.023X10^23 molecules, and is commonly calculated as the sum of the atomic weights of each atom in a molecule. A Molar Solution is an aqueous solution consisting of one mole of a substance plus enough water to make one Liter of solution. A Molal Solution is an aqueous solution consisting of one mole of a substance plus 1 kg of water (usually very close to 1 L water). The total volume may thus be more than 1 L. The difference between molar and molal is important when a solution contains a large amount of non-aqueous substance. For example, cream has 20% fat (homogenized in very small droplets). There would be a 20% difference between the molarity of salt and its molality. A lab might report a Na concentration of 20 mMolar (moles/L cream), but really all the Na would be in the 80% that was water. It's "real" concentration (molality) would be 24 mMolal (moles/L water). Concentrations of ions are often given in Equivalents (or milliequivalents, mEq) per Liter. The equivalents of an ion is equal to the molarity times the number of charges per molecule. Thus Equivalents is the measure of CHARGE concentration. Osmoles refers to the number of impermeable particles dissolved in a solution, regardless of charge. This will be important for determining the diffusional movement of water. For substances that maintain their molecular structure when they dissolve (e.g. glucose), the osmolarity and the molarity are essentially the same. For substances that dissociate when they dissolve, the osmolarity is the number of free particles times the molarity. Thus for a pure NaCl solution, a 1 Molar solution would be 2 Osmolar (1 for Na, and 1 for Cl). When measured as osmoles per liter, one obtains the osmolarity. For osmoles per kg water, one obtains osmolality. |
|
Copyright 1999, Joe Patlak, Department of Physiology, University
of Vermont. |
