Molecular weight by Lands Berger’s method

Molecular weight by Lands Berger’s method-

source : Department of Natural Sciences

Molecular weight by Lands Berger’s method:

This method is used for the determination of molecular weight of a nonvolatile substance by elevation in boiling point technique.

The Apparatus used in this method is as follows-

source : you tube

It consists of following parts :

a) Boiling flask:-

               It is a simple round bottom flask fitted with a safety tube and a delivery tube. Flask contains pure solvent. Solvent is heated externally in this flask by burner and solvent vapours are sent to boiling tube through rose head. The function of rose head is to distribute the vapours of solvent throughout the solvent uniformly in the boiling tube.

b) Boiling tube:-

           It is a graduated good quality glass tube. It has a bulb with a hole in the side tube . It is fitted with a delivery tube and a very sensitive thermometer called Beckmann thermometer.

c) Beckmann thermometer :-

         It is fitted in the boiling tube. It is a very sensitive thermometer. It consists of a large bulb connected by a fine capillary tube with a bulb at the top, which can accommodate excess of Hg. This thermometer measures small differences in temperature upto 1/100 ⁰ C

d) Outer Jacket :-

        The boiling tube is surrounded by a jacket . It collects the uncondensed hot vapours coming from the boiling tube through the hole. It protects the boiling tube from surroundings and prevents the heat loss by radiation.  These vapours are led to a condenser connected at the bottom of the jacket.

Procedure:-

                    About 10 ml of pure solvent is taken in the boiling tube and the rose head tube is connected to the boiling flask which also contain pure solvent . The boiling flask is heated then solvent vapours pass in the solvent taken in the boiling tube. The latent heat of condensation raises the temperature of the solvent. when solvent begins to boil the temperature of the thermometer becomes constant.  It is noted ( T₁ ).

                    The apparatus is then allowed to cool. A small quantity of solute (nearly 0.2 gm) is dissolved in the solvent present in the boiling tube. The apparatus is refitted and vapours of solvent are again led into it and the boiling point of solution is recorded in the same manner (T₂ ). The difference of two boiling points (i.e T₂ – T₁) gives the  Δ T_b  i.e elevation in boiling point . The volume of solution in the boiling tube is also recorded.

                   Since the solution is very dilute , the mass of solvent i.e ‘W’ can be taken to be equal to the volume of the solution multiplied by density of the solvent. By knowing the values of w , W, Δ T_b and k_b , the molecular  mass of solute ‘m’ can be calculated by the formula

m = 1000 k_b . w / Δ T_{b .}W

Limitations :-

This ebullioscopic method used for the determination of molecular weight of a nonvolatile  substance have following limitations:

1)  The solution should be dilute.

2) The molecular state of the solute should not change in the solution. i.e solute should neither dissociate nor associate in the solution.