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Grade 12th PassPhysical Chemistry

The vapour pressure of a pure liquid A is 10 torr at the same temperature when,1 g of B is dissolved in 20 g of A,the vapour pressure is reduced by 1 torr. If the mol. wt.of A is 200 u then the molecular weight of B is:\

Profile image of ASh
8 Years agoGrade 12th Pass
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1 Answer

Profile image of Ashish Jangir
8 Years ago
Given,
PA = 10 torr {vapour pressure of pure A}
Decrease in vapour pressure = 1torr
so total vapour pressure after adding B is 9 torr and B is non volatile substance therefore PB=0
then according to Dalton’s law of partial pressure-
P= XA.P+ XB.P
9 =  {(mA/MA) / (mA/MA + mB/MB)}.10 + 0           [0 because P= 0]
mA(mass of A)=20gm, MA=200,  mB=1gm ,  MB=??
put the values and u will get MB=90u