Question

what evolution in boiling point of alcohol is to be expected when 5g of urea ( mol. Mass =60) are dissolved in 75 g of it? The molal elevation constant for alcohol is 1.15°C?​

Answer 1

given, mass of Urea = 5g

molar mass of urea = 60g/mol

so, number of mole of urea = 5/60 = 1/12

mass of solvent (alcohol) = 75g

we know, molality = number of mole of solute/mass of solvent in kg

= (1/12) × 1000/75

= 1000/(12 × 75)

= 1.11molal....(1)

now using formula, ∆Tb = Kb × m

where ∆Tb is elevation in boiling point , Kb is elevation constant and m is molality.

given, Kb = 1.15 °C/molal , m =1.11 molal

so, ∆Tb = 1.15 × 1.11 = 1.2765°C

hence, elevation in boiling point is 1.2765°C