Answer to Question #168806 in Organic Chemistry for Juan

Question

A certain substance X condenses at a temperature of 147.2 °C. But if a 800. g sample of X is prepared with 77.9 g of urea ((NH2)2Co) dissolved in it, the

sample is found to have a condensation point of 150.7 °C instead. Calculate the molal boiling point elevation constant Kb of X.

Be sure your answer has the correct number of significant digits 

Answer 1

Mass of urea= 77.9g

Molar mass of urea= 60g/mol

Moles of urea= mass/molar mass = 77.9/60= 1.298mol

Molality of urea=moles of urea/Kg of solvent

= 1.298mol/0.800Kg

= 1.623Molal

Now, we can now find the boiling point elevation constant K_b by using the relation:

∆T_b= K_bm

Where ∆T_b is the boiling point elevation, m is the molality of solute.

∆T_b= T₂-T₁= K_bm

150.7°C - 147.2°C = K_b x 1.623

K_b= 3.5/1.623=2.157