98 THE THEORY OF IONIZATIONcharged radicals — the ions — which move through the solu-
tion and carry the current to the electrodes. The question arises
as to how these charged particles affect the freezing point. Ions
of opposite charge exert an enormous attraction for each other,
and this attraction would be expected to impede their freedom of
motion. On the other hand, each ion in the interior of the solu-
tion is surrounded by ions of opposite charge, and the resultant
force of strong ionic attraction from all directions is zero. Hence
at the other extreme we might expect the motion of each ion to
be as unrestricted as the motion of an uncharged molecule.
The actual measurement of the freezing-point lowering caused
by electrolytes has led us to choose the second postulate, that
is, of the freedom of motion of the charged ions, as the nearer ap-
proximation to the true condition within the solution.
If we proceed, then, in the assumption that charged ions have
the same effect on the freezing point as uncharged molecules, we
can again regard the freezing-point lowering as proportional to the
total number of moles in a given solution, only we must revise our
definition of mole so as to include moles of ions as well as moles
of non-electrolytes. For example, if 1 formula weight of AB is
50 per cent ionized into A+ B ~ we have 0.5 mole of AB, 0.5 mole
of A+ and 0.5 mole of B", or in all 1.5 moles, and the effect of AB
on the freezing point would be 1.5 times what we would expect
if AB were a non-electrolyte.
The following set of problems will test our grasp of the principle
discussed in this section.
- Assuming complete ionization of the electrolyte, what
would be the freezing point of 10 grams of NaCl in 500 grams
of water? - — of 10 grams of CaCl2 in 500 grams of water?
- — of 10 grams of FeCl 3 in 500 grams of water?
- — of 10 grams of K,Fe(CN) 6 in 500 grams of water?
16-19. What is the osmotic pressure at 0° of the salt in
Questions 12-15, inclusive?- What is the osmotic pressure of 0.1 mole of NaCl in
1,000 grams of water if the salt is 86 per cent ionized? - — of 0.1 mole of BaCl 2 in 1,000 grams of water if the
salt is 72 per cent ionized?