Elevation of boiling point
- Increase in temperature increases the vapour pressure of a
- A liquid boils at atemperature where the vapour pressure is equal to the atmospheric
- Vapourpressure of the solvent decreases in the presence of non-volatile solute.
- The boilingpoint of a solution is always higher than that ofthe boiling point of the pure solvent in which thesolution is prepared as.
- The elevation ofboiling point also depends on the number ofsolute molecules rather than their nature.
- The increase in the boiling point = ΔTb = Tb – Tb0. This is known as elevation of boiling point.
Tb0 = boiling point of pure solvent
Tb0 = boiling point of solution
- In dilute solutions the elevation of boiling point Tb is directly proportional to the molal concentration of the solute in a solution. Mathematically,
ΔTb ∝ m
ΔTb = kb m
m = Molality = number of moles of solute dissolved in 1 kg of solvent
Kb = constant of proportionality. It is also known as Boiling Point Elevation Constant or Molal Elevation Constant (Ebullioscopic Constant). The unit of Kb is K kg mol-1.
- 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.
