PHYSICAL CHEMISTRY IN BRIEF

CHAP. 2: STATE BEHAVIOUR [CONTENTS] 60

2.4.3 Ideal mixture

A mixture that obeys Amagat’s law at all temperatures and pressures is defined as anideal
mixture. It often serves as a standard state used to calculate thermodynamic quantities of
real mixtures. [see6.1].

Note:Amagat’s law can be applied to both gaseous and liquid mixtures, and to mixtures

in the solid state.

2.4.4 Pseudocritical quantities

The theorem of corresponding states2.2.9can be also used for mixtures if we substitute the
critical quantities bypseudocritical quantities. Pseudocritical quantities of a mixture are
calculated from the critical quantities of its components. The simplest relations were suggested
by Kay:

Tc′=

∑k

j=1

xjTc,j, p′c=

∑k

j=1

xjpc,j, Vc′=

∑k

j=1

xjRTc,j/pc,j. (2.41)

Example

The critical temperature and pressure of substance A are given asTc= 400K,pc= 4MPa, and

those of substance B asTc= 200K,pc = 6MPa. Calculate the pseudocritical temperature,

pseudocritical pressure and pseudocritical volume of a mixture containing 40 moles percent of

substance A.

Solution

We substitute into equations (2.41), where the mole fraction of substance A is 0.4, and that of

substance B is 0.6.

Tc′= 0. 4 ×400 + 0. 6 ×200 = 280K,

p′c= 0. 4 ×4 + 0. 6 ×6 = 5. 2 MPa,

Vc′= 0. 4

8. 314 × 400

4

+ 0. 6 ×

8. 314 × 200

2

= 498. 84 cm^3 mol−^1.