Solutions - Basic Definitions and Concepts

The pure substances from which a solution can be made are caUed the components, or constituents of a solution. The extensive properties of a solution are determined by the pressure, temperature, and the amount of each constituent. The intensive properties of a solution are determined by the pressure, temperature and the relative amounts of each constituent, or in other words by the pressure, temperature and composition of the solution. For aqueous solutions, the most commonly used measurement of composition of the solution is the molality, m. Molality is defined as the number of moles of a solute in one kilogram of the solvent, and for aqueous solutions the solvent is water. One of the advantages of using the molality scsde for concentration is that it is independent of temperature 

1000gi/(M go or 1000 hj/g where gj and go are the number of grams of solute and solvent, M is the solute molecule weight and nj is the number of gm-moles of solute. 

The thermodynamic analysis of solutions is facilitated by the introduction of quantities that measure how the extensive thermodynamic quantities (V, E, H, G,. ..) of the system depend on the state variables T, P, and nj. This leads to the definition of partial molar quantities where, if we let Y be any extensive thermodynamic property, we can define the partial molar value of Y for the ith component as  

Partial molar quantities are intensive properties of the solution since they depend only on the composition of the solution, not upon the total amount d each component. If we add the several components simultaneously, keeping their ratios constant, the partial molal quantities remain the same. We can thus integrate the above expression keeping nj, n2,. .. in constant proportions and find, while holding temperature and pressure constant, that 

Since Y represents any extensive property, if we substitute the Gibbs free energy. G, for Y we obtain the Gibbs-Duhero equation 

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