The molal boiling point elevation constant Kb= 1.26·°C·kgmol−1 for a certain substance X .When 36.1g of urea ((NH2)2CO)) are dissolved in 550.g of X, the solution boils at 109.1°C .Calculate the boiling point of pure X. ?

The boiling point of pure X is 107.73 °C

Explanation:

Step 1: Data given

molal boiling point elevation constant Kb = 1.26 °C /m

Mass of urea = 36.1 grams

Mass of X = 550 grams = 0.550 Kg

Solution boils at 109.1 °C

Molar mass of urea = 60.06 g/mol

Step 2: Calculate moles urea

Moles urea = mass urea / molar mass urea

Moles urea = 36.1 grams / 60.06 g/mol

Moles urea = 0.601 moles

Step 3: Calculate boiling point elevation

ΔT = i*Kb*m

⇒with ΔT = the boiling point elevation = TO BE DETERMINED

⇒with i = the van't Hoff factor for urea = 1

⇒with Kb = the molal boiling point elevation constant = 1.26 °C /m

⇒with m = the molality = moles urea / mass X = 0.601 moles / 0.550 kg = 1.09 molal

ΔT = 1.26 °C/m * 1.09 m

ΔT  = 1.37°C

Step 4: Calculate boiling point of pure X

ΔT = Tb (solution) - Tb (pure solvent)

1.37°C = 109.1 °C - Tb(pure solvent)

Tb pure solvent = 109.1 - 1.37 °C  = 107.73 °C

The boiling point of pure X is 107.73 °C

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