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Lesson_14

# Lesson_14 - Boiling point elevation(Tb Lesson 14 Boiling...

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Lesson 14 Boiling point elevation ( T b )

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Lesson 14 Boiling point elevation The normal boiling point of a pure liquid is the temperature at which the vapour pressure reaches 1 atm According to Raoult’s law, the vapour pressure of a liquid/solid solution is less than that of the pure liquid Therefore, the temperature of the solution might be increased to make it boil (i.e. to make the vapour pressure reach 1 atm)
Lesson 14 Boiling point elevation At the normal boiling point: S T P curve the of slope b = - = 1 ( 29 + = = = = - = 2 1 2 2 2 1 , 2 1 1 1 n n n S S x S atm x S P x S P T vap b P 1

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Lesson 14 Boling point elevation For very dilute solutions (where Raoult’s law is more applicable), n 1 >> n 2 and the previous equation can be simplified to: 1 1 2 2 1 2 / / 1 1 M m M m S n n S T b = = Since S and M 1 are properties of the pure solvent, they can be combined in a single constant K b : S kg g M K b ) / 1000 ( 1 = Substitute into the equation for T b :
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