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Significance and Units of the van der Waals’constants
The significance and the units of the constant ‘a’ and ‘b’ used in the van derWaals equation are discussed as van der Waals’ constant ‘a’ The constant ‘a’ isrelated to the magnitude of the attractive forces among the molecules in aparticular gas. Thus, greater the value of ‘a’ more will be the attractiveforces. It may be noted that ‘a’ is higher for gases like NH3, HCl and SO2 which can be easilyliquefied than for gases like He, H2 andN2 which 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 ‘a’ can be calculated as follows:
Units of ‘a’ are related to the units in which pressure ‘p’ andvolume ‘V’ are expressed.
• Pressure is in atmospheres and volume in litres, a = atm. L2 mol-2
• Pressure is in atmospheres and volume in m3,a = atm m6 mol-2
• Pressure is in Nm”2 and volume in m3, a = Nm-2 m6 mol-2
Van der Waals’ constant ‘b’. The constant ‘b’ 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 ‘h’ canbe calculated as follows :
Units of ‘b’ may be L mol-1 or m3 mol-1.