' g of a non-volatile electrolyte solid solute of molar mass 
when dissolved in 100 mL water, decreases vapour pressure of water from 640 mm Hg to 600 mm Hg. If aqueous solution of the electrolyte boils at 375 K and 
for water is 
, then the mole fraction of the electrolyte solute ( 
) in the solution can be expressed as
(Given : density of water 
and boiling point of water 
)
Text Solution
Verified by ExpertsD
Given the vapour pressure of pure water 
and vapour pressure of solution 
.
According to Raoult's law for an electrolyte solution, the relative lowering of vapour pressure is given by 
, where 
is the van't Hoff factor and 
is the mole fraction of the solute.
Substituting the values: 
…… .. (i)
The elevation in boiling point is given by 
, where 
is the molality.
Given 
and 
, so 
.
Molality 
(since mass of water 
).
Substituting in the elevation formula: 
……… (ii)
Substitute the value of 
from (ii) into (i):
This can be rewritten as 
.
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