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In thermodynamics, the ebullioscopic constant Kb relates molality b to boiling point elevation.[1] It is the ratio of the latter to the former:
Δ T b = i K b b {\displaystyle \Delta T_{\text{b}}=iK_{\text{b}}b} i is the van 't Hoff factor, the number of particles the solute splits into or forms when dissolved. b is the molality of the solution.
i is the van 't Hoff factor, the number of particles the solute splits into or forms when dissolved.
b is the molality of the solution.
A formula to compute the ebullioscopic constant is:[2]
K b = R M T b 2 1000 Δ H vap {\displaystyle K_{\text{b}}={\frac {RMT_{\text{b}}^{2}}{1000\Delta H_{\text{vap}}}}} R is the ideal gas constant. M is the molar mass of the solvent. Tb is boiling point of the pure solvent in kelvin. ΔHvap is the molar enthalpy of vaporization of the solvent.
R is the ideal gas constant.
M is the molar mass of the solvent.
Tb is boiling point of the pure solvent in kelvin.
ΔHvap 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 ΔT, depends on the number of particles dissolved into the solvent and not the nature of those particles.
Values for some solvents[edit] Solvent[3] Kb (in K⋅kg/mol) Acetic acid 3.08 Benzene 2.53 Camphor 5.95 Carbon disulfide 2.34 Carbon tetrachloride 5.03 Chloroform 3.63 Cyclohexane 2.79 Diethyl ether 2.02 Ethanol 1.07 Water 0.512 See also[edit] Ebullioscope List of boiling and freezing information of solvents Boiling-point elevation Colligative properties References[edit] ^ "Ebullioscopic Constant". CHEMISTRY GLOSSARY. ^ Martin, Chaplin. "Colligative Properties". London South Bank University. London South Bank University. Archived from the original on 2020-08-13. Retrieved 2015-01-13. ^ P. W. Atkins, Physical Chemistry, 4th Ed., p. C17 (Table 7.2) External links[edit] Ebullioscopic constant calculator Archived 2024-03-02 at the Wayback Machine
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