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SOLUTIONSMANUAL TO ACCOMPANY ATKINS' PHYSICAL CHEMISTRY 153
For MgCu2 the percentage is
100 × 1 × (24.31 gmol−1)
1 × (24.31 gmol−1) + 2 × (63.55 gmol−1)
= 16
For Mg2Cu the percentage by mass of Mg is 43.
For the isopleth at 25% Mg at high temperatures the initially system corre-
sponds to a single-phase liquid system. At a1 (at about 730 ○C) MgCu2 begins
to come out of solution and the liquid becomes richer in Mg, moving toward
e2. At a2 there is solidMgCu2 + liquid of composition e2 (33 per cent bymass of
Mg).�is solution freezes without further change.�e cooling curve is shown
next to the phase diagram.
P5D.6 �e schematic phase diagram is shown in Fig 5.26. �e solid points are the
data given in the Exercise, and the lines are simply plausible connections be-
tween these points; the open circle is the incongruent melting of K2FeCl4. An
expanded section of the phase diagram is shown as this includes the part of
interest.
0.3 0.4 0.5 0.6 0.7
300
350
400
450
500
xFeCl2
θ/
○ C
K2FeCl4 KFeCl3
Figure 5.26
�e compounds KFeCl3 and K2FeCl4 correspond to xFeCl2 = 1
2 and xFeCl2 =
1
3 ,
respectively; these compositions are shown on the diagram.
When a melt of composition xFeCl2 = 0.36 is cooled (the dotted line), solid is
�rst deposited when the temperature falls to around 360 ○C.�e solid consists
of K2FeCl4, and as the temperature falls further the liquid grows progressively
richer in FeCl2.�is process continues until the temperature falls to 350 ○C, at
which point the liquid has the same composition as the eutectic and the whole
sample solidi�es to a mixture of K2FeCl4 and KFeCl3.