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Chemistry · Glossary

What is Partial molar quantity?

Also known as: partial molar volume

Definition 3.1 University Chemistry — Year 2 · Chapter 3 — Chemical Potential and Mixtures

Let X(T,p,n1,…,nk)X(T, p, n_1, \dots, n_k) be an extensive state function of a phase containing the amounts n1,…,nkn_1, \dots, n_k. The partial molar quantity of constituent ii is

Xˉi=(∂X∂ni)T,p,nj≠i,\bar X_i = \left(\frac{\partial X}{\partial n_i}\right)_{T, p, n_{j \ne i}} ,

the change of XX per mole of ii added to a large amount of the mixture at fixed TT, pp and other amounts. For X=VX = V it is the partial molar volume Vˉi\bar V_i.

The tangent construction. The molar volume of a model binary mixture (blue) lies below the straight line joining the pure volumes: mixing contracts. The tangent at a composition cuts the two edges at the partial molar volumes V_1 and V_2 of that composition.
The tangent construction. The molar volume of a model binary mixture (blue) lies below the straight line joining the pure volumes: mixing contracts. The tangent at a composition cuts the two edges at the partial molar volumes Vˉ1\bar V_1 and Vˉ2\bar V_2 of that composition.
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