Question

Calculate the elevation constant kb for water and the boiling point of 0.1 molar urea solution. Latent heat of vaporization of water is 9.72kcalmol1 at 373.15k

Answer 1

using formula

Kb = RT²/1000L

where R is universal gas constant.

T is temperature in Kelvin.

L is latent heat of vaporisation.

given, T = 373.15K

latent heat of vaporisation, L = 9.72 Kcal/g = 9720 cal/mol

mass of 1 mole of water , M = 18g

so, latent heat of vaporisation, L = 9720/18 cal/g = 540 cal/g

and R = 2cal/mol.K

now, kb = (2cal/mol.k × 373.15²K²)/(1000 × 540 cal/g)

= (2 × 373.15²/540000) g.K/mol

= 0.515 g.k/mol

now ∆Tb = kb × m

= 0.515 × 0.1

= 0.0515 K

so, boiling point of urea solution is (373.15 + 0.0515)K

= 373.2015K