Q26 / 97 · NTA Abhyas (JEE Main) 2020 · Chemistry · Solutions

The vapour pressure of a pure liquid A is 70 torr at 27°C . It forms an ideal solution with another liquid B. The mole fraction of B is 0.2 and total vapour pressure of the solution in 84 torr at 27°C . The vapour pressure of pure liquid B at 27°C is

  • Correct answer
    A
    140 torr
  • B
    56 torr
  • C
    14 torr
  • D
    70 torr

Solution

According to Raoult's law, p=pAo.XA+pBo.XB

Here pAo=70 torr,XA=1-XB=1-0.2=0.8,p=84 torr, XB=0.2 and pBo=?

So 84 torr =70 torr×0.8+pBo×0.2

or pBo=84 torr-56 torr0.2=140 torr .

Hence, the correct answer is option: 'A'.

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