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Molecular wt is not a colligative property but using colligative properties we can calculate mol wt here we have to assume that solute niether dissociates nor associates in soln ,in both the cases no of particles will change,colligative property depend upon no of particles so it will give wrong mol mass i.e abnormal mol mass secondly Colligative property is inversely proportional to molmass &directly prop to no of particles so if no of partl icles increases C.P will increase &mol mass observed will be less than actuall mol mass for e.g in ur case 1 NaCl on dissolving in water ionise to form 2 particles value of C.P obtained acc to 2 particles so mol mass dec to half of its real value. if we wish to get correct mol mass we have to introduce a corrective factor ,vant hoff factor i which is=no of particals after association or dissociation/no of particals beforeasso ordissociation for eg in case of NaCl i=2 so  Tb=ikbx m where Tb is elevation of B.P which is colligative property
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E-Store
dilute solutions???
