Question
Sun June 26, 2011 By: Asif Ansari
 

2.175 gm of non volatile solute is added in 39 gm of organic liquid.The lowering in the vapour pressure is 40 mm oh Hg. Calculate molar weight of solute>molecular weight of organic liquid is 78.Assuming solution to be dilute vapour pressure of pure liquid is 640mm of Hg.

Expert Reply
Mon June 27, 2011
Mass of non volatile solute(WB) = 2.175 g
Mass of organic liquid (WA) =39 g
 Given lowering in the vapour pressure ( P0A-PA) = 40 mm of Hg
Vapour pressure of  pure liquid, P0A= 640 mm of Hg

molar mass of organic liquid = 78 g mol-1

For dilute solution,

(P0A-P/P0A  = (WB XMA) /(MB XWA)
   40/640 = (2.175 X78)/(MB X39)
MB =69.6 g mol-1
Therefore molar weight of solute =69.6 g mol-1
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