Question

Calculate the molality of solution of nonvolatile solute if boiling point of solution, molal elevation constant for solvent are \(319.8 \text{K}\) and \(2.5 \text{K kg mol}^{-1}\) respectively [Boiling point of solvent = \(319.5 \text{K}\)]

• A. \(0.12 \text{mol kg}^{-1}\)
• B. \(0.14 \text{mol kg}^{-1}\)
• C. \(0.17 \text{mol kg}^{-1}\)
• D. \(0.21 \text{mol kg}^{-1}\)

Hint:

Find delta Tb as the difference of boiling points, then divide by Kb.

Answer 1

The Correct Option is A

Step 1: Temperature rise:
The solution boils at $319.8$ K while the pure solvent boils at $319.5$ K, so the rise is $0.3$ K.

Step 2: Divide:
$m = \frac{0.3\ \text{K}}{2.5\ \text{K kg mol}^{-1}} = 0.12$ mol/kg.

Final Answer:
Molality $= 0.12$ mol/kg, option (A). \[ \boxed{0.12\ \text{mol kg}^{-1}} \]