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: Understanding the Concept:
A nonvolatile solute raises the boiling point of a solvent. The elevation is proportional to the molality: \(\Delta T_b = K_b m\).

Step 2: Key Formula or Approach:
\(\Delta T_b = T_b(\text{solution}) - T_b(\text{solvent})\), then \(m = \frac{\Delta T_b}{K_b}\).

Step 3: Detailed Explanation:
\(\Delta T_b = 319.8 - 319.5 = 0.3\) K.
\[ m = \frac{0.3}{2.5} = 0.12\ \text{mol kg}^{-1} \]
Options B, C and D would give \(\Delta T_b\) of \(0.35\), \(0.425\) and \(0.525\) K, which do not match the data.

Final Answer:
The molality is \(0.12\) mol/kg, option (A). \[ \boxed{0.12\ \text{mol kg}^{-1}} \]