Question

Which of the following is the correct order of their increasing boiling points?

• A. \( 10^{-4} \, \text{M NaCl} > 10^{-3} \, \text{M MgCl}_2 > 10^{-2} \, \text{M NaCl} > 10^{-4} \, \text{M urea} \)
• B. \( 10^{-2} \, \text{M NaCl} > 10^{-3} \, \text{M MgCl}_2 > 10^{-4} \, \text{M NaCl} > 10^{-4} \, \text{M urea} \)
• C. \( 10^{-4} \, \text{M urea} > 10^{-2} \, \text{M NaCl} > 10^{-3} \, \text{M MgCl}_2 > 10^{-4} \, \text{M NaCl} \)
• D. \( 10^{-4} \, \text{M NaCl} > 10^{-3} \, \text{M MgCl}_2 > 10^{-4} \, \text{M urea} > 10^{-2} \, \text{M NaCl} \)

Hint:

Boiling point elevation depends on the dissociation of solute particles, so ionic compounds elevate the boiling point more than non-electrolytes.

Answer 1

The Correct Option is B

Boiling point elevation is directly proportional to the number of ions present in the solution, as described by the formula: \[ \Delta T_b = i K_b m \] where:
- \(i\) is the van 't Hoff factor (number of ions),
- \(K_b\) is the ebullioscopic constant, and
- \(m\) is the molality.
Since \(NaCl\) dissociates into two ions (Na\(^+\) and Cl\(^-\)), and \(MgCl_2\) dissociates into three ions (Mg\(^{2+}\) and 2Cl\(^-\)), it follows that the boiling point elevation will be greatest for \(MgCl_2\) followed by \(NaCl\), and the least for urea, which does not dissociate. Thus, the correct answer is Option (B).