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Grade 12Physical Chemistry

Vapour pressure of pure water at 25 celsius is 80 torr and that of an aqueous solution os 75 torr

Question image for Vapour pressure of pure water at 25 celsius is 80
Profile image of Simon
4 Years agoGrade 12
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1 Answer

Profile image of Anish
4 Years ago
Let M be the molecular weight of solute.
 
Number of moles of solute = 
M
5.40
 
 
Number of moles of water = 
18
90.0
 =5
 
Mole fraction of solute = 
M
5.40
 +5
M
5.40
 
 = 
5.40+5M
5.40
 
 
Relative lowering in the vapour pressure = 
0
 
0
 −P
 = 
23.76
23.76−23.32
 =0.01852
 
The relative lowering in the vapour pressure of the solution is equal to the mole fraction of solute. 
 
0.01852= 
5.40+5M
5.40
 
 
⟹5.40+5M=291.6
 
5M=286.2
 
⟹M=57.24 g/mol