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What weight of the non-volatile solute, urea (NH2 – CO – NH2) needs to be dissolved in 100g fo water, in order to decrease the vapour pressure of water by 25%? What will be the molality of the solution?

Shane Macguire , 10 Years ago
Grade upto college level
anser 2 Answers
Deepak Patra

Last Activity: 10 Years ago

Hello Student,
Please find the answer to your question
Po– P/Po = w/3 /w/m+W/M
Here, w and m are wt. and molecular wt. of solute, W and M are wt. and molecular weight of solvent
p = pressure of solution;
po = Normal vapour pressure
Let the initial (normal) pressure (po) = p
∴ Pressure of solution = 75/100 * p = 3/4 p
M = 60, M = 118, W = 100 gm
∴ p-3/4 p/p = w/60 /w/60 + 100/18
1/4 = w/60 /(w/60) + 5.55
Or 4w/60 = w/60 + 5.55
3w/60 = w/20 = 5.55
Or w = 111 g
Molality = No. of moles of solute/Wt. of solvent * 1000
= 111 *1000/60 *100 = 18.52 m

Thanks
Deepak patra
askIITians Faculty

Rishi Sharma

Last Activity: 4 Years ago

Dear Student,
Please find below in the attached image the solution to your problem.

Thanks and Regards
645-1985_3.PNG

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