Question -
Answer -
Significance and Units of the van der WaalsтАЩconstants
The significance and the units of the constant тАШaтАЩ and тАШbтАЩ used in the van derWaals equation are discussed as van der WaalsтАЩ constant тАШaтАЩ The constant тАШaтАЩ isrelated to the magnitude of the attractive forces among the molecules in aparticular gas. Thus, greater the value of тАШaтАЩ more will be the attractiveforces. It may be noted that тАШaтАЩ is higher for gases like NH3, HCl and SO2┬аwhich can be easilyliquefied than for gases like He, H2┬аandN2┬аwhich areliquefied with diffieculty. Actually the former are of polar nature and theattractive forces in their molecules are higher than the latter gases which areof non-polar nature. The units of the constant тАШaтАЩ can be calculated as follows:
Units of тАШaтАЩ are related to the units in which pressure тАШpтАЩ andvolume тАШVтАЩ are expressed.
тАв Pressure is in atmospheres and volume in litres, a = atm. L2┬аmol-2
тАв Pressure is in atmospheres and volume in m3,a = atm m6┬аmol-2
тАв Pressure is in NmтАЭ2 and volume in m3, a = Nm-2┬аm6┬аmol-2
Van der WaalsтАЩ constant тАШbтАЩ. The constant тАШbтАЩ represents the co-volume orexcluded volume which is effective volume of the molecules in a gas. It is, infact, four times the volume occupied by the gas molecules. The units of тАШhтАЩ canbe calculated as follows :
Units of тАШbтАЩ may be L mol-1┬аor m3┬аmol-1.