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Molecular Weight (MW) Determination By Ebulliometry

• An ebulliometer was first used to determine the molecular weights of substances, by measuring the changes in the boiling point of water cause by the presence of the unknown substance.
  
  This method has now well and truly been superceded by mass spectrometry.
  
  
• An ebulliometer, or ebullioscope, is able to determine molecular weights of non-ionic and non volatile substances in solution, regardless of the solute involved.
  
  
• This is because regardless of the substance involved, 1 mole of the solute added to 1 L of water will raise the boiling point by 0.52^oC.
  
  
• If 500g of an unknown substance dissolved in 1L of water, raises the boiling point by 0.76 ^oC, then -
  
  To find out how many moles of the substance we have -
  
  
  

  				mole
  moles		=		--------		*		0.76oC = 1.46 mole (of substance)
  				0.52

  
  
  To find out the molecular weight (MW) of the substance -
  
  
  

  500 g

  mole * MW (Daltons)

  MW (Daltons)

  =

  --------------

  *

  ----------------------------

  =

  342 Daltons

  1.46 mole

  g

  
  
  This MW correspondence to the MW of sucrose and could be confirmed with further tests.
  
  
• The property of substances to be able to affect the physical properties of water equally, is termed colligative property.
  
  
• These physical properties affected equally in this way, include the boiling and freezing point, and osmotic and vapour pressure of water.