SOLUTIONS 1 7 



DaltorCs Laic. — In a gaseous mixture, the total pressure 

 is equal to the sum of the pressures which each gas would exert 

 if it alone filled the whole of the receptacle. 



Pressure proportional to Molecular Concentration. — The 

 above laws are completely independent of the chemical nature 

 of the gas, they depend only on the number of gaseous 

 molecules in a given space, i.e. on the molecular concentration. 

 If we double the mass of the gas in a given space, we double 

 the number of molecules, and we also double the pressure, 

 whatever the nature of the molecules. We may also double 

 the pressure by compressing the molecules of a gas, or of 

 several gases, into a space half the original size. The 

 molecular concentration of a gas, or of a mixture of gases, 

 is the ratio of the number of molecules to the volume they 

 occupy. The pressure of a gas or of a mixture of gases 

 is proportional to its molecular concentration. This is a 

 better and a shorter way of expressing both Boyle's law 

 and Dalton's law. 



One gramme-molecule of a gas, whatever its nature, con- 

 densed into the volume of 1 litre, has a pressure of 22'35 

 atmospheres. Similarly one gramme-molecule of a solute, 

 whatever its nature, when dissolved in a litre of water, has 

 the same pressure, viz. 22'35 atmospheres. 



Absolute Zero. — According to Gay-Lussac's law, the volume 

 of a gas diminishes by -h-J-^ of its volume at 0° ( '. for each 

 degree fall of temperature. Thus if the contraction is the 

 same for all temperatures, the volume would be reduced to 

 zero at —273° C. This is the absolute zero of temperature. 

 Temperatures measured from this point are called absolute 

 temperatures, and are designated by the symbol T. If t° 

 indicates the Centigrade temperature above the freezing point 

 of water, then the absolute temperature is equal to t° + S27!i°. 



The Gaseous Constant. — Consider a mass of gas at 0° C. 

 under a pressure P o , with volume V . At the absolute 

 temperature T, if the pressure be unaltered, the volume of 



this gas will be — °— . Therefore the constant PV, the product 



O / ( s * 



of the pressure by the volume, will be represented by — °^°— . 



