Given,
Pure vapour pressure of A , p∘A=180 mmPure vapour pressure of B , p∘B=100 mm
Mole fraction of A , XA=0.5∴Mole fraction of B, XB=1−0.5=0.5
By Raoult's law
Total vapour pressure of solution,
PT=XA.p∘A+XB.p∘B
PT=180×0.5+100×0.5
=140 mm.
PT>XA.p∘A+XB.p∘B
200 mm>140 mm
Thus, the given mixture shows positive deviation from the Raoult's law and forms minimum boiling azeotrope