# 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

Or,

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.

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.

Solution

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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