ebullioscopic

Ebullioscopic refers to methods and measurements that rely on boiling point elevation, particularly the ebullioscopic method used to determine molar masses and related properties of solutes in a solvent. The technique is a classic example of colligative properties, where the effect depends on the number of dissolved particles rather than their identity.

Principle: When a nonvolatile solute is dissolved in a solvent, the boiling point of the solution rises relative to the pure solvent. The elevation is given by ΔTb = i Kb m, where ΔTb is the boiling-point increase, i is the van’t Hoff factor (reflecting solute dissociation or association), Kb is the ebullioscopic constant of the solvent, and m is the molality of the solution. For many organic and aqueous systems, i is close to 1 for non-electrolytes. The observed boiling point is Tb(solution) = Tb(solvent) + ΔTb.

Method and calculations: In an ebullioscopic experiment, a known mass of solvent is heated to its boiling point, and a known amount of solute is dissolved. The boiling point of the resulting solution is measured with an ebullioscope. From the measured ΔTb and the known solvent constants, m can be calculated, and, for a given solvent mass, the molar mass of the solute can be derived (assuming a simple, nonvolatile solute). For electrolytes, the value of i must be considered.

Applications and limitations: Ebullioscopy is used to estimate molar masses and assess solution purity in systems where the solute is nonvolatile. Limitations include the need for accurate boiling-point measurements, control of pressure, and complications from volatile solutes, strong electrolytes, or significant solute–solvent interactions that violate ideal behavior.

See also: Cryoscopy, Osmometry, Colligative properties, Molality.