Which of the following ions will contribute most to elevating the boiling point of H_2O when added at equal molal concentrations?

• A. Na^+
• B. Ca^{2+}
• C. Cl^-
• D. Al^{3+}

Answer: (D)

Verified step by step guidance

1. Understand that the boiling point elevation of a solution depends on the colligative property called boiling point elevation, which is related to the number of solute particles in the solution, not their identity.
2. Recall the formula for boiling point elevation: \(\Delta T_b = i \cdot K_b \cdot m\), where \(\Delta T_b\) is the boiling point elevation, \(i\) is the van't Hoff factor (number of particles the solute dissociates into), \(K_b\) is the ebullioscopic constant of the solvent, and \(m\) is the molality of the solution.
3. Determine the van't Hoff factor \(i\) for each ion when dissolved in water: for example, \(Na^+\) dissociates into 1 particle, \(Ca^{2+}\) dissociates into 1 particle (as a cation) but usually comes with counterions, and \(Al^{3+}\) dissociates into more particles due to its charge and associated counterions.
4. Since the problem states equal molal concentrations of ions, compare the effective number of particles contributed by each ion in solution, considering their charge and dissociation behavior; ions with higher charge typically come from salts that dissociate into more ions, increasing \(i\) and thus elevating the boiling point more.
5. Conclude that \(Al^{3+}\) ions contribute the most to boiling point elevation because they come from salts that dissociate into more particles (higher \(i\)), leading to a greater increase in boiling point compared to \(Na^+\), \(Ca^{2+}\), or \(Cl^-\) at the same molality.