Difference between revisions of "Boiling-point elevation"
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'''Boiling-point elevation''' is the increase in temperature of the [[boiling point]] of a liquid that contains non-volatile solute. It is a [[colligative property]]: for dilute solutions of non-[[electrolyte]]s, the difference in boiling point from that of the pure solvent is proportional to the [[Amount of substance|amount]] of solute. | '''Boiling-point elevation''' is the increase in temperature of the [[boiling point]] of a liquid that contains non-volatile solute. It is a [[colligative property]]: for dilute solutions of non-[[electrolyte]]s, the difference in boiling point from that of the pure solvent is proportional to the [[Amount of substance|amount]] of solute. | ||
| − | '''Ebullioscopy''' is the experimental technique that uses boiling-point elevation to measure the [[molecular weight]] of compounds. | + | '''Ebullioscopy''' is the experimental technique that uses boiling-point elevation to measure the [[molecular weight]] of compounds. '''Tonometry''' is a related technique that directly measures the reduction in [[vapour pressure]] of the solution compared to that of the pure solvent. Both techniques are of historical interest only. |
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| + | ==Description== | ||
| + | The boiling-point elevation Δ''T''<sub>b</sub> is proportional to the [[molality]] of the solution ''m'': the proportionality constant is called the '''ebullioscopic constant''' ''K''<sub>eb</sub>. | ||
| + | :Δ''T''<sub>b</sub> = ''K''<sub>eb</sub>''m'' | ||
| + | The molality of the solution is the amount of solute divided by the mass of solvent. For dilute solutions, where the mass of the solution can be approximated by the mass of solvent, the molality is equal to the [[mass fraction]] ''w'' of solute divided by its [[molar mass]] ''M''. Hence, the molar mass of a solute can be calculated from the boiling-point elevation by | ||
| + | :''M'' = ''K''<sub>eb</sub>''w''/Δ''T''<sub>b</sub> | ||
| + | |||
| + | ==See also== | ||
| + | *[[Freezing-point depression]] | ||
==Notes and references== | ==Notes and references== | ||
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[[Category:Chemical properties]] | [[Category:Chemical properties]] | ||
[[Category:Practical techniques]] | [[Category:Practical techniques]] | ||
| + | [[Category:Phase transitions]] | ||
{{CC-BY-3.0}} | {{CC-BY-3.0}} | ||
Revision as of 05:50, 27 March 2011
Boiling-point elevation is the increase in temperature of the boiling point of a liquid that contains non-volatile solute. It is a colligative property: for dilute solutions of non-electrolytes, the difference in boiling point from that of the pure solvent is proportional to the amount of solute.
Ebullioscopy is the experimental technique that uses boiling-point elevation to measure the molecular weight of compounds. Tonometry is a related technique that directly measures the reduction in vapour pressure of the solution compared to that of the pure solvent. Both techniques are of historical interest only.
Description
The boiling-point elevation ΔTb is proportional to the molality of the solution m: the proportionality constant is called the ebullioscopic constant Keb.
- ΔTb = Kebm
The molality of the solution is the amount of solute divided by the mass of solvent. For dilute solutions, where the mass of the solution can be approximated by the mass of solvent, the molality is equal to the mass fraction w of solute divided by its molar mass M. Hence, the molar mass of a solute can be calculated from the boiling-point elevation by
- M = Kebw/ΔTb
See also
Notes and references
Notes
References
External links
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See also the corresponding article on Wikipedia. |
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