Page:Outlines of Physical Chemistry - 1899.djvu/26

 6 OUTLINES OF PHYSICAL CHEMISTEY

If we introduce this value into the equation, we get : pv =p v [l + ±(t - 273)]=p v (l + £L-1)

T PqVq

For any given mass of gas, the value of =^-P is a con-

stant usually represented by r, so that we can give the

general equation the simple form :

N.B. — The numerical value of r depends, on the unit of volume and unit of pressure chosen. We shall return to this subject.

Determination of the density of a Gas or Vapour

Having given a certain weight g of a gas, We have to

observe :

v, its volume in cubic centimetres ;

p, its pressure in millimetres oi mercury ;

t, its temperature in degrees Centigrade.

We have then all the data necessary for the calculation of the density of the gas, relatively to air.

Since p v = p v (1 + a t),

therefore v = — -J-—-

��Po(l + at)

will be the volume of the gas at 0° and under normal pressure. The same volume of air would weigh

0-00129 P v, x grams,

since one cubic centimetre of air at 0° and 760 millimetres pressure weighs 0*00129 grams.

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