Molecular Weight Of A Substance By Cryoscopic Method

Determination of The Molecular Weight Of A Substance By Cryoscopic Method

Please Download the full report of the Determination of the molecular weight of a substance by cryoscopic method

Please Download the full report of the Determination of the molecular weight of a substance by cryoscopic method

Theory of Determination of Molecular Weight Of A Substance By Cryoscopic Method :

The property of a solvent is differ from those of the pure solvents due to interactions that take place between the solute and solvent molecules. The properties that exhibit such changes are called the colligative properties. The colligative properties are-

  • Vapour pressure lowering
  • Boiling point elevation
  • Freezing point depression
  • Change in Osmotic pressure etc.

The properties are only depends on the number of particles ( molecules or ions ) that are dissolved in the solvent This determination of molecular weight of a substance by cryoscopic method will explain the phenomenon of the Freezing point depression. When the a particular solute is dissolve in solvents, the following expression holds true:

ΔT= T0f – Tf = Kf  m = Kf (1000 w1/M w2 )

The trams T0f  and  Tf   refers to the freezing point temperature of the pure solvent and the solution respectively. The  tram “m” refers to the  molality Kf refers to the freezing point depression constant. Its only depends on the solvents. (For Cyclohexane Kf = 20.2 deg.kg/mole of solute.)

 

molecular weight of a substance by cryoscopic method
Freezing point depression

Apparatus and Chemicals needed for determination of the molecular weight of a substance by cryoscopic method:

  • Digital thermometer (-100C to 1000C )
  • Stirring Rod
  • Ice
  • Stop Watch
  • Cyclohexane
  • Camphor etc.

Procedure for determination of the molecular weight of a substance by cryoscopic method:

  • Set up the apparatus ensuring that the stirrer does not touch the thermometer.
  • Take 20 to 25 mL cyclohexane into the tube and place in the air jacket and dip it in the freezing mixture. Cool it by direct immersion in the ice bath until the cyclohexane starts to freeze.
  • Take thermometer reading at every 30 second.
  • Stir the cyclohexane slowly and and take the thermometer reading until the constant reading is obtained.
  • Take out the freezing tube and warm up with hand until the solid melts.
  • Add measured quantity of (about 0.5 to 0.6 gm ) of the solute into the tube and dissolve it bye stirring.
  • Now again take the thermometer reading until the cyclohexane freeze and constant thermometer reading is obtained and Plot temperature against time for solvent and solute in the same graph.
  • Now calculate the molecular weight of a substance by cryoscopic method.

Please Download the full report of the Determination of the molecular weight of a substance by cryoscopic method

Please Download the full report of the Determination of the molecular weight of a substance by cryoscopic method

For your help we give screen shot of the taken data sheet and the plot:

Data:

1.Data for the pure cyclohexane  –

 

molecular weight of a substance by cryoscopic method
Data for the pure cyclohexane

2. Data for cyclohexane after adding the solute-

 

 

molecular weight of a substance by cryoscopic method
Data for cyclohexane after adding the solute

3. The plot for the above two data tables-

 

 

 

molecular weight of a substance by cryoscopic method
The plot for the above two data tables

Please Download the full report of the Determination of the molecular weight of a substance by cryoscopic method

Please Download the full report of the Determination of the molecular weight of a substance by cryoscopic method

Calculation :

  1. Volume of the solvent = 25 gm
  2. Density of the solvent = 0.79 gm/mL
  3.  So Weight of the solvent = (25 × 0.79) gm = 19.79gm
  4. Weight of the solute = 0.50gm
  5. Freezing point depression constant Kf = 20.2 deg.kg/mole
  6. Freezing point depression from the graph = (6.5-4.5)0C = 2.00C

Result:

  • The molecular weight of the naphthalene is M= (1000×Kf ×w1)/(w2 × ΔT)

=(1000×20.2×0.5)/(19.79× 2.0 )

=114.13g/mol

The actual molecular weight of the naphthalene 128.0 gm/mol

So the % of error ={(128.0-114.13)/128}×100%

=10.83%

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