Apparent Molar Property Information
An apparent molar property is a quantity that can be used to calculate a property of a solution. For instance, the volume of the solution is given by
where Vsolvent is the molar volume of the solvent, nsolvent is the number of moles of solvent, is the apparent molar volume of solute i, and ni is the number of moles of solute i in the solution. (The apparent molar volume can also be denoted Vφ.) This equation applied to a single-solute solution serves as the definition of the apparent molar volume of the solute. (For multi-solute solutions, the equation does not give an unambiguous definition of the apparent molar properties.)
Apparent molar properties are not in fact constants (even at a given temperature), but are functions of the composition. At infinite dilution, they are equal to the partial molar property.
Some apparent molar properties that are commonly used are apparent molar enthalpy, heat capacity, and volume.
The apparent molar volume of a salt is usually less than the molar volume of the solid salt. For instance, solid NaCl has a volume of 27 cc per mole, but the apparent molar volume at low concentrations is only 16.6 cc/mole. In fact, some aqueous electrolytes have negative apparent molar volumes: NaOH -6.7, LiOH -6.0, and Na2CO3 -6.7 cc/mole.[1] This means that their solutions in a given amount of water have a smaller volume than the same amount of pure water. The physical reason is that nearby water molecules are strongly attracted to the ions so that they occupy less space.
See also
References
- ^ Herbert Harned and Benton Owen, The Physical Chemistry of Electrolytic Solutions, 1950, p. 253.
External links
Categories: Physical chemistry | Thermodynamics
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