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.

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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