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 mm Hg at `25^(@)`C. Calculate the molecular weight of the solute

Options

A. 74.2
B. 75.6
C. 67.83
D. 78.7

Answer

C

Explanation

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`