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CGP EDU Academic Team
Published on: August 13, 2026
What weight of the non-volatile solute urea (NH2—CO—NH2) needs to be dissolved in 100g of water, in order to decrease the vapour-pressure of water by 25% ? What will be the molality of the solution ?
Text Solution
Verified by ExpertsThe correct answer is:
75
Let the vapour-pressure of water = 100
∴ Vapour-pressure of urea solution = 75
By Raoult’s law
= 
.....(1)
In equation(1) w (wt. of urea) = ?
m(mol. wt. of urea) = 60
W(wt. of water) = 100 g
M(mol. wt. of water) = 18 g
P0(V.P of water) = 100
Ps(V.P. of solution) = 75
So, 
= 
On solving we get, w = 111.1 g
Molality of urea solution
= 
= 
= 18.5 M
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