Vapour pressure of pure A is 70 mm of Hg at 25oC.It forms an ideal solution with 'B' in which mole fraction of A is 0.8. If the vapour pressure of the solution is 84 mm of Hg at 25oC the vapour pressure of pure B at 25oC is

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56 mm

70 mm

140 mm

28 mm

answer is C.

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Detailed Solution

Given,p∘A=70mmHg xA=0.8∴ xB=(1−0.8)=0.2 and p=84mmHgAccording to Raoult's law, p=pA∘xA+pB∘xB84=0.8×70+0.2×pB∘pB∘=280.2=140mmHg

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