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Question

# 100 g of liquid A (molar mass 140 g mol−1) was dissolved in 1000 g of liquid B (molar mass 180 g mol−1). The vapour pressure of pure liquid B was found to be 500 torr. Calculate the vapour pressure of pure liquid A and its vapour pressure in the solution if the total vapour pressure of the solution is 475 torr.

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Solution

## Number of mole is the ratio of mass to molar mass.For liquid A, number of moles =100140=0.714For liquid B, number of moles =1000180=5.556Mole fraction of A =0.7140.714+5.556=0.114Mole fraction of B =1−0.114=0.886Ptotal=PA+PB=PoAXA+PoBXB475=PoA×0.114+500×0.886475=0.114PoA+443Vapour pressure of pure A, PoA=280.7 torrVapour pressure of A in solution =280.7×0.114=32 torrThus vapour pressure of pure solute A is 280.7 torr and its vapour pressure in solution is 32 torr

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