Ebullioscopic constant

{{Short description|Chemical and physical constant of materials}}

{{Use American English|date = March 2019}}

{{More citations needed|date=June 2016}}

In thermodynamics, the ebullioscopic constant {{math|K_b}} relates molality {{mvar|b}} to boiling point elevation.{{Cite web|url = http://glossary.periodni.com/glossary.php?en=ebullioscopic+constant|title = Ebullioscopic Constant|website = CHEMISTRY GLOSSARY|publisher = }} It is the ratio of the latter to the former:

:$\backslash Delta\; T\_\backslash text\{b\}\; =\; iK\_\backslash text\{b\}\; b$

• {{mvar|i}} is the van 't Hoff factor, the number of particles the solute splits into or forms when dissolved.
• {{mvar|b}} is the molality of the solution.

A formula to compute the ebullioscopic constant is:{{Cite web|url = http://www1.lsbu.ac.uk/water/colligative_properties.html#ebu|title = Colligative Properties|website = London South Bank University|publisher = London South Bank University|last = Martin|first = Chaplin}}

:$K\_\backslash text\{b\}\; =\; \backslash frac\{RMT\_\backslash text\{b\}^2\}\{1000\backslash Delta\; H\_\backslash text\{vap\}\}$

• {{mvar|R}} is the ideal gas constant.
• {{mvar|M}} is the molar mass of the solvent.
• {{math|T_b}} is boiling point of the pure solvent in kelvin.
• {{math|ΔH_vap}} is the molar enthalpy of vaporization of the solvent.

Through the procedure called ebullioscopy, a known constant can be used to calculate an unknown molar mass. The term ebullioscopy means "boiling measurement" in Latin. This is related to cryoscopy, which determines the same value from the cryoscopic constant (of freezing point depression).

This property of elevation of boiling point is a colligative property. It means that the property, in this case {{math|ΔT}}, depends on the number of particles dissolved into the solvent and not the nature of those particles.