o
Let w2 and M2 = Masses and molar masses of solute w1 = Mass of solvent
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.
Sol. Elevation of boiling point ΔTb =( 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:
=( 0.37 x 342 x 500) /( 0.52 x 1000) = 121.67 g( approximately) Hence, 121.67 g of sucrose is to be added.