18 GENERAL BIOCHEMISTRY 



that depends upon the velocities of the molecules, their sizes, and their 

 number. 



Studies of gases have disclosed quantitative relationships in their 

 behavior. For example, if the temperature of a given sample of gas is 

 kept constant, an application of external pressure compresses the gas 

 and its volume decreases. On the other hand, when the volume of this 

 sample is kept constant and the temperature is raised, the pressure ex- 

 erted by the gas increases. It is known that molecules travel faster as 

 the temperature increases. Hence they collide more often with the 

 walls of the container and exert an increased pressure. Furthermore, 

 if the number of gas particles is increased by increasing the size of the 

 sample, the number of collisions per unit area and the pressure are 

 increased. These effects can be written mathematically as propor- 

 tionalities: 



1 

 P oc — (constant n, T) 



where P ^ pressure, V ^ volume, T ^ absolute temperature, and 

 n ^ number of moles of gas ^ weight divided by molecular weight. 

 And 



Poz T (constant n, V) 

 P oz n (constant V, T) 



These expressions may be combined to 



and incorporation of a proportionality constant R called the gas con- 

 stant yields the ideal gas law 



p = ^y^ or PV = nRT 



The constant R possesses the same value for all gases or mixtures of 

 gases and once evaluated may be used in all calculations with the 

 ideal gas law. Indeed this same constant appears in many other ex- 

 pressions employed with other types of systems. 



It is customary to use the word "ideal" in connection with this law 

 because common gases do not obey it exactly. In fact the deviations 

 can be quite striking. Nevertheless, the ideal gas law is useful and 

 proves satisfactory for most purposes when {a) the pressure is relatively 

 low and (h) the temperature is rather far above the boiling (liquefy- 

 ing) point of the gas involved. Thus the ideal gas law is readily ap- 

 plied to gases that boil far below room temperature and to other gases 



