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At 25^(@)C, the vapour pressure of pure ...

At `25^(@)C`, the vapour pressure of pure water is `25.0 mm Hg`. And that of an aqueous dilute solution of urea is `20 mm Hg`. Calculate the molality of the solution.

Text Solution

Verified by Experts

The given values are,
`P_(A)^(@) = 25 mm Hg` , `P_(S) = 22 mm Hg`
`m = ?`
Using Raoult's law
`(P_(A)^(@) - P_(S)) / P_(A)^(@) = (chi_(B))` Substituting all the values
`chi_(B) = (25- 20) /25 = 0.20`
`:.chi_(A) = 1 - chi_(B) = 1 - 0.2 = 0.8`
The realationship between molality and mole fraction of solute in a dilute solution is
`m = (chi_(B))/(chi _(A)Mw_(A))xx1000` ,where `Mw_(A)` = molar mass of solvent
`(0.2 xx 1000)/(0.8 xx 18) = 13.8`
Molality of solution is `13.8`.
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