Page:Outlines of Physical Chemistry - 1899.djvu/58

 88 OUTLINES OF PHYSICAL CHEMISTRY

Let us now apply the general equation of a gas to the gram-molecule and see what value the constant r will assume. We have already put this equation in the form :

pv =™^° t or pv = RT.

Therefore r represents |^°.

For one gram-molecule of a gas :

v = 22880 cubic centimetres at 0° and under the pressure p.

p = 760 millimetres of mercury = 1088 grams pressure per square centimetre of surface.

Therefore

^ 22880 x 1088 QAAflK t

R = 278 = 84685 '

In order to arrive at this value for r, the volume v

must be measured in cubic centimetres, and the pressure p

in grams per square centimetre of surface.

Specific Heat of Gases

The determination of the specific heat of gases is almost always made under constant pressure.

It is extremely difficult to experimentally determine the specific heat under constant volume. However, the ratio between the two specific heats has been determined for different gases (Clement and Desormes 1 s method, Laplace's method).

For atmospheric air the ratio 2 is 1*405, and Mayer has been able to deduce the mechanical equivalent of heat from this.

1 The number 84685 is very often made use of in physico-chemical calculations. It sometimes has an abstract signification ; but more often it has a concrete meaning, and represents centimetre-grams of work or an equivalent quantity of some other form of energy (heat, electricity, <fcc).

2 This means that for air the ratio

Specific heat ( constant pressure ) \*aqk Specific heat (constant volume) For oxygen the ratio is 1*404, for nitrogen 1*410, for ammonia 1*30,

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