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Question

In 100g of napthalene,.2.423g of S was dissolved. Melting point of napthalene=80.1°C. ∆Tf=661°C, Lf=35.7cal/g of napthalene. Molecular formula of sulphur added is
(a)S2.   (b)S4.   (c)S6.   (d)S8


Solution

The molecular formula of S added is S8.

Tf = 80 C = 353.1 K
∆Tf = 0.661K
W = weight of solute = 2.423 g
w = weight of solvent = 100 g
Lf =35.7 cal/g
the formula for Kf  is
 Kf = RTf^2/1000Lf 

also
M =Kf x W/w x ∆Tf
 =( RTf^2/1000Lf ) x W/ w x ∆Tf      ( substituting the value of Kf)

= [2 x (353.1)^2 x 2.423]/[1000 x 35.7 x 0.661 x 100]
= 256g/mol

Now atomic weight of S = 32g/mol
Let the molecular formula be Sx
then x = 256/32 = 8
so formula is S8
 

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