Experiment 15


To determine the molecular weight of the given substance by Rast method.


The molecular weight of a non-volatile substance is determined by Rast method employing the principle of depression of freezing point. The following equation is applicable.




where m = mol. wt. of the solute, Kf = molar depression constant of the solvent, w = weight of the solute in gram, W = weight of the solvent in gram and ∆Tf = depression in freezing point of the solvent.


  1. A small Pyrex or Corning tube.
  2. Sulphuric acid or liquid paraffin or glycerine bath.
  3. Thin glass rod.
  4. Thiele's tube.
  5. Thermometer with the calibration ...

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