1.

A solution containing 30 gms of non-volatile solute in exactly 90 gm water has a vapour pressure of 21.85 mm Hg at 25^(@)C. Further 18 gms of water is then added to the solution. The resulting solution has a vapour pressure of 22.15 mmHg at 25^(@)C. Calculatethe molecular weight of the solute

Answer»

<P>74.2
75.6
67.83
78.7

Solution :We have,
`(p^(@)-21.85)/(21.85)=(30xx18)/(90xx m)`, for 1 case…….(i)
WT. of solvent `= 90+18=108 gm`
`(p^(@)-22.15)/(22.15)=(30xx18)/(108xx m)`, for II case …..(ii)
By EQ. (1) `p^(@)m-21.85m=21.85xx6=131.1`
By eq. (2) `p^(@)m-22.15m =22.15xx5=110.75`
`0.30m=20.35`
`m=(20.35)/(0.30)=67.83`


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