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Relative lowering of vapour pressure from the elevation of boiling point

This type of question, asked 4 times: 4 Apr 2024, 21 Jan 2026 S2, 24 Jan 2026 S1, 4 Apr 2026 S1. All 4 questions below, oldest first.

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  1. Question 1

    4 Apr 2024

    58
    2.5 g2.5\text{ }g of a non-volatile, non-electrolyte is dissolved in 100 g100\text{ }g of water at 25∘C25^{\circ}C. The solution showed a boiling point elevation by 2∘C2^{\circ}C. Assuming the solute concentration in negligible with respect to the solvent concentration, the vapour pressure of the resulting aqueous solution is ____ mmmm of HgHg (nearest integer) [Given: Molal boiling point elevation constant of water (Kb)=0.52 K.kgmol−1\left( K_{b} \right)= 0.52\text{ }K.kgmol^{- 1}, 1 atm1\text{ }atm pressure =760 mm= 760\text{ }mm of HgHg, molar mass of water  =18 g mol−1]\left. \ = 18\text{ }g{\text{ }mol}^{- 1} \right\rbrack

  2. Question 2

    21 Jan 2026 S2

    48
    A substance ' X ' ( 1.5 g ) dissolved in 150 g of a solvent ' Y ' (molar mass =300 g mol−1= 300\text{ }g{\text{ }mol}^{- 1} ) led to an elevation of the boiling point by 0.5 K . The relative lowering in the vapour pressure of the solvent ' Y ' is ____\_\_\_\_ ×10−2\times 10^{- 2}. (Nearest integer) [Given : KbK_{b} of the solvent =5.0 K kg mol−1= 5.0\text{ }K\text{ }kg{\text{ }mol}^{- 1} ] Assume the solution to be dilute and no association or dissociation of X takes place in solution.

  3. Question 3

    24 Jan 2026 S1

    37
    ' W ' g of a non-volatile electrolyte solid solute of molar mass ' M ' gmol−1gmol^{- 1} when dissolved in 100 mL water, decreases vapour pressure of water from 640 mm Hg to 600 mm Hg . If aqueous solution of the electrolyte boils at 375 K and KbK_{b} for water is 0.52 K kg mol−10.52\text{ }K\text{ }kg{\text{ }mol}^{- 1}, then the mole fraction of the electrolyte solute (x2x_{2}) in the solution can be expressed as (Given : density of water =1 g/mL= 1\text{ }g/mL and boiling point of water =373 K= 373\text{ }K )

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  4. Question 4

    4 Apr 2026 S1

    50
    A non-volatile, non-electrolyte solid solute when dissolved in 40 g of a solvent, the vapor pressure of the solvent decreased from 760 mm Hg to 750 mm Hg . If the same solution boils at 320 K , then the number of moles of the solvent present in the solution is ____\_\_\_\_ . (Nearest integer) [Given : boiling point of the pure solvent =319.5 K, Kb= 319.5\text{ }K,{\text{ }K}_{b} of the solvent =0.3 K kg mol−1= 0.3\text{ }K\text{ }kg{\text{ }mol}^{- 1} ]