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185
jment No.
DETERMINATION OF MOLECULAR
RASTS WI. BY
METHOD
sim
Jo determine the molecular mas: F
ethod. of the given compund by Rasts
principle
hen a non volati is di :
wi i ne ye jal re solute is dissolved, the freezing point of the sol-
vent is lowered. The difference of the freezing point of the pure solvent
and the solution is referred to as the Depression of freezing point and it is
related to the mlecular mass of the non volatile solute as
AT = 1000Ksx w
Mx W
where AT = Depression of Freezing point
Kr = Molar depression constant
w = Weight of solute
W = Weight of solvent
M = Molecular weight of the solute
If the molal depression constant Kf of the solvent is known, we can
find out the mol. mass of an unknown solute by finding out the depression
in freezing point caused by dissolving a known weight of the substance in
aknown weight of the solvent.
Requirements
1. Dry boiling tube
2. Sensitive thermometer
3, Stirrer and two holed cork
4, Naphthalene or Camphor
Procedure
About 10 g of Napthalene is accurately weighed and ——
the dry boiling tube fitted with a sensitive thermometer an‘ stirrer using
the two holed cork. The assembly is heated to melt the Naphthalene and heating continued till the tempeature is about 5-6° C above melting Point,
The assembly is transferred to a cooling chamber and allowed to cool
down slowly. A conical flask(or Horlics bottle) filled with shreds of
thermocool can be used as the cooling chamber or air jacket,
Temperatutures are recorded every 15 seconds interval and continued till
the temperature is below 2 - 3° C, the freezing point of naphthalene. A
graph is plotted with temperature and time as axes. The temperature which
remains steady for some time (the horizontal portion of the cooling curve)
is the freezing point of Napthalene.
About 0.2 g (w1) of the unknown substance is accurately weighed
and introduced into the boiling tube. It is heated to melt (stirred well) and
the freezing point is determined. Another addition of about 0.2 g (w2) of
the solute is also done and the freezing point is again determined from its
cooling curve. :
Observations
Napthalene
Time 0 15 30 45 60 75
[Time
he
Solution II - [Naphthalene + (w, + w2) g solute]
