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150 5 SIMPLEMIXTURES
105
110
115
120
125
130
135
(i) (ii)
(iii)
(iv) (v)
time
T
/K
Figure 5.21
a b
T1
T2
time
te
m
pe
ra
tu
re
Figure 5.22
E5D.6(b) Figure 5.23 shows the phase diagram to which dotted horizontal lines have
added at the relevant temperatures.
(i) At 500 ○C the phase diagram shows a single liquid phase at all composi-
tions, so B is soluble in A in all proportions .
(ii) At 390 ○C solid B exists in equilibriumwith a liquid whose composition is
circled and labelled x1 in the �gure.�at composition is xB = x1 = 0.63 .
(iii) At 300 ○C the solubility of AB2 in B is indicated by the point x2. At this
point two phases coexist: solid AB2 and a liquid mixture of A and B with
mole fraction xB = x2 = 0.41. Although the liquid does not contain any
AB2 units, the liquid can be thought of as a mixture of dissociated AB2
in A. Let the amount in moles of the compound be nc and that of free
A be na. �us, the amount of A (regardless of whether free or in the
compound) is nA = na + nc and the amount of B is nB = 2nc. �e mole
fraction of B is
xB = x2 =
nB
nA + nB
= 2nc
(na + nc) + 2nc
= 2nc
na + 3nc