Question

Boiling point of water at 750 mm Hg is 99.63°C. How much sucrose is to be added to 500 g of water such that it boils at 100°C. Molal elevation constant for water is 0.52 K kg mol−¹ . 

Answer 1

Here, elevation of boiling point ∆Tb = (100 + 273) − (99.63 + 273) = 0.37 K

 Mass of water, wl = 500 g

 Molar mass of sucrose (C₁₂H₂₂O₁₁₎, M₂ = 11 × 12 + 22 × 1 + 11 × 16

 = 342 g mol⁻¹

 Molal elevation constant, Kb = 0.52 K kg mol⁻¹ We know that: 

= 121.67 g (approximately)

 Hence, 121.67 g of sucrose is to be added.

 Note: There is a slight variation in this answer and the one given in the NCERT textbook