The vapour pressure lowering caused by the addition of 100 g of sucrose (molecular mass = 342) to 1000 g of water (if the vapour pressure of water at 250C is 23.8 mm Hg) is (AFMC 2012)

Number of moles of C₁₂H₂₂O₁₁ 100 342 0.292mol Number of moles of H₂O 1000 18 =55.5 mol ( Molecular mass of sucrose = 342), (Molecular mass of H₂O = 18) Vapour pressure of pure water o p 23.8mmHg According to Raoult’s law o p n n N p p 0.292 23.8 0.292 55.5 >> 23.8 0.292 p 0.125mm Hg 55.79 x