Question

The molal boiling point elevation constant Kb = 1.97 C .Kg.Mol-1 for a certain substance X. When 31.98 g of urea are dissolved in 650.G of X. The solution boils at 137.0 C. Calculate the boiling point of pure X

Answer 1

Answer:

Boiling point of pure liquid is 135.4°C

Explanation:

The boiling point elevation (Colligative property of solutions) follows the formula:

ΔT = Kb×m×i

ΔT is equal to increase in boiling point (Boililng point solution - boiling point pure liquid), Kb is boiling point elevation constant of the liquid, m is molality of solution and i is Van't Hoff factor.

Molality of solution (Moles urea / Kg X):

Moles urea: 31.98g * (1mol / 60.06g) = 0.53247 moles

-60.06g/mol = Molar mass urea-

In 650g = 0.650kg:

0.53247 moles urea / 0.650kg = 0.8192m

Van't Hoff factor of urea is 1

Replacing in the equation:

ΔT = Kb×m×i

ΔT = 1.97°CKgMol⁻¹×0.8192m×1

ΔT = 1.6

As the solution boils at 137°C:

ΔT = Boililng point solution - boiling point pure liquid

ΔT = 1.6°C = 137.0°C - boiling point pure liquid

Boiling point of pure liquid is 135.4°C