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Boiling point formula


What is Boiling point?

The boiling point is the temperature at which a liquid starts to boil and transforms to vapor. The liquid begins to boil at a particular temperature when its vapor pressure becomes equal to atmospheric pressure. The presence of solute lowers the vapor pressure and makes it necessary to heat at a higher temperature to boil the solution. The boiling point of water (universal solvent) is known to be 100oC, which states that water starts to turn to a vapor state, and its vapor pressure becomes equal to atmospheric pressure at 100oC.


Boiling point formula

The boiling point elevation is directly proportional to the molality of the solute particles in the solution.

∆Tb = Kbm

∆Tb = Amount by which the boiling point is elevated.

Kb = molal boiling-point elevation constant which is solvent dependent.

m =molality (moles of solute particles per kg of solution)

Boiling point of solution is given by

Boiling point  =  boiling point of the solvent + ∆Tb


T solution = Tsolvent +  ∆T


Example 1

The boiling point of a solvent is 70oC, the values for Kb and m are 0.5oC/m and 0.015m. Determine the boiling point of the solution.


The parameters given are

m = 0.015 and Kb = 0.5oC/m

Water is the solvent, therefore

Tsolvent = 70oC

The boiling point formula is given by

Tsolution = Tsolvent +∆T

∆T = Kbm

∆T = 0.5 x 0.015 = 0.0075oC

Tsolution = 70 + 0.0075 = 70.0075oC


Example 2

Determine the boiling point of the solution whose m = 0.0285m and Kb = 0.612oC/m? Water is the solvent here.


The given parameters are,

M = 0.0285m and Kb = 0.612oC/m

Since water is solvent,

Tsolvent = 100oC

Boiling point formula is

Tsolution = Tsolvent + ∆T

∆T = Kbm

∆T = 0.612 x 0.0285 = 0.017442

Tsolution = 100 + 0.017442

= 100.017442oC


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