The height A of the column in r, since the pressure per square centimetre must be p grams, is given by —
s MS
The vapour pressure over the solution in r has now to be found. The decrease of pressure from the surface of the liquid in iS to that in r is equal to the weight of a column of vapour h cm. high and of 1 sq. cm. cross-section. If p be the vapour pressure at the lower and p that at the higher surface, this weight is equal to ^ — p\
Now we know that the weight M of a gram-molecule of
a gas at pressure p is contained in the volume v = — . M
grams occupy v c.c, and consequently 1 c.c. weighs -if grams,
and h c.c. weigh -M grams. The weight of a column of
vapour of 1 sq. cm. section between the two liquid surfaces
is, therefore, equal to jp - p* and to -M.
If we now substitute the values found above for h and r, we obtain —
V MN p N or — ^ ^ =
p iV
The Relative Lowering of Vapour Pressure. — The
relationship just deduced may be brought into a very simple
form. ^ ""^ is called the relative lowering of vapour
P pressure, and it obviously gives the difference between the
vapour pressure of solvent and that of the solution referred to
the vapour pressure of the solvent. It is independent of the
temperature, of the nature of the solvent, and of the nature
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