Question

The molal boiling point elevation constant Kb= 2.13 ℃kgmo-for a certain substance X, when 12. g of urea are dissolved in 100. g of X, the solution boils at 126.3 °C. Calculate the boiling point of pure X. Be sure your answer has the correct number of significant digits.

Answer 1

Answer: The boiling point of pure solution is 122.04°C

Explanation:

Elevation in boiling point is defined as the difference in the boiling point of solution and freezing point of pure solution.

The equation used to calculate elevation in boiling point follows:

$\Delta T_b=\text{Boiling point of solution}-\text{Boiling point of pure solution}$

To calculate the elevation in boiling point, we use the equation:

$\Delta T_b=iK_bm$

Or,

$text{Boiling point of solution}-\text{Boiling point of pure solution}=i\times K_b\times \frac{m_{solute}\times 1000}{M_{solute}\times W_{solvent}\text{ in grams}}$

where,

Boiling point of solution = 126.3°C

i = Vant hoff factor = 1 (For non-electrolytes)

$K_b$ = molal boiling point elevation constant = 2.13°C/m

$m_{solute}$ = Given mass of solute (urea) = 12. g

$M_{solute}$ = Molar mass of solute (urea) = 60 g/mol

$W_{solvent}$ = Mass of solvent (X) = 100. g

Putting values in above equation, we get:

$126.3^oC-\text{Boiling point of pure solution}=1\times 2.13^oC/m\times \frac{12.\times 1000}{60g/mol\times 100.}\\\\\text{Boiling point of pure solution}=122.04^oC$

Hence, the boiling point of pure solution is 122.04°C