Boiling Point Formula

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Boiling Point Elevation Formula

One of the important properties of any solution is the boiling point elevation. The elevation in boiling point formula indicates that increase in the boiling point can be calculated by the use of boiling point elevation and the molality and constant of the solution. The vapour of any solvent can decrease when a solute is added. And by this, some of the solvent molecules can be replaced by a solute. A high temperature is required to match the atmospheric pressure and the vapour pressure.



The formula of boiling points has the following variables:

TB = boiling point

To = boiling temperature

R = ideal gas constant

P = vapour pressure of the liquid

P0 = pressure at boiling point

^Vap = heat of vaporization 

Boiling Point Equation

We all know that water boils at 100 degrees at 1atm pressure but if we add a little amount of salt in it, then an interesting thing happens, it increases the boiling point of the solution. This is proven by researchers that, adding a solute in a solution changes its form and results in an elevation in the boiling point. Both the amount of the change and the present boiling point are proportional to each other. 

The addition of a solution decreases the vapour pressure of the given solvent. This change happens due to the displacement of the solvent molecules. This formula for elevation in boiling point hence proves that few of the solvent molecules that are present on the surface of the liquid solution are replaced by the solvent that is both non-electrolyte and electrolyte. If the amount of the solvent molecules in a surface is lowered then less evaporation can occur. In order to balance the vapour and make it equal to the ambient pressure, a higher boiling point is observed. 

How to Find the Boiling Point of a Solution 

The boiling point of a solution can be found by the following steps of the formula of elevation in boiling point:

  1. ^Tb = 1000*Kb*wM*W where, “is the weight of the solute”, “is the molar mass of the solute and “W” is the weight of the solvent in grams.

  2. ^Tb = 1000 *Kb*wM*W

  3. 1.1 = 1000 *2.53 *10M*200

As a result of adding solvent to a solute, the vapour pressure of the individual solvent becomes less than the vapour pressure above the pure solvent present. This results in the increase in the boiling point of the solvent and it will be needed to treat at a higher temperature in order to make vapour pressure equal to the external pressure. 

Hence the boiling point of the solution changes as the concentration of that particular solute in the solution changes its form. 

Boiling Point Elevation Equation: Non-volatile Solutes

The boiling point of a solvent above any solution changes its form and becomes greater than the boiling point of the solvent irrespective of non-volatile or volatile solute. But to maintain its simplicity, only the non-volatile solute shall be considered over here. The formula is:

^T = Kbm

Where: ^T is the change in the boiling point of the solvent,

Kb is the molal boiling point elevation constant, and

M is the molal concentration of the solute in the solution.

The molal boiling point elevation constant, Kb has a specific value depending on the identity of the solution. 

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FAQ (Frequently Asked Questions)

Q1. Why Does Boiling Point Elevation Occur?

Ans. The boiling point of any liquid is its temperature in which the vapour pressure is equal to the pressure of the surrounding environment. Any non-volatile substance does not undergo evaporation readily and they have a low vapour pressure. Addition of the non-volatile solute to solvent results in lowering the vapour pressure of the solution. Hence, a larger amount of solute should be supplied to the solution in order to make it boil. This increase in the boiling temperature results in the elevation of the boiling point.

Q2. How Does Molality Affect the Boiling Point?

Ans. The boiling point of a solvent increases whenever a non-volatile substance is dissolved in it. It is said that the higher the concentration, the higher is the boiling point. This acts as a dissolved solute and crowds out other solvent molecules at its surface. It is difficult for the solvent molecules to escape into the gas phase if the concentration of the solute is high although, the rate of the condensation from gas to liquid form is not affected. So the solvent molecules need a higher temperature in order to escape the boiling at atmospheric pressure.