Solubility laws and vapor pressure


The solubility of a solvent or of the solute is not same always. There are a number of factors that promote or regress solubility.  When a substance is mixed with a solvent, there are several possible results. The determining factor for the level of solubility of the substance is defined as the maximum possible concentration of the solute.  Solubility and the capacity of a solvent to get dissolved follows the solubility rules and the solubility rules help to determine the extent of the solubility of a given. Some of the laws governing the solubility are as follows.


Dilution law       

According to this law, if we dilute a solution by adding a solvent into it, the amount of solute remains constant throughout the process.  This can be represented as:

M1V1= M2V2

Also, N1V1= N2V2 and Normality =z× Molarity


Both normality and molarity of a solution are a function of temperature while mass percentage, parts per million, molality and mole fraction are independent of temperature.



Vapor pressure

Vapor pressure is a liquid property related to evaporation. Molecules have some kinetic energy in the liquid or any substance, the distribution of kinetic energies is related to the temperature of the given system. As the temperature of liquid increases, the kinetic energy of its molecules also increases. The vapour pressure is increased by the rise in the number of molecules transitioning into a vapour while there is also increase in the kinetic energy of the moleculesIn simple terms, the vapour pressure of a liquid or solution refers to the pressure exerted by the vapour in equilibrium with the liquid or solution at a given temperature. Vapour pressure is directly proportional to the escaping tendency. 



Raoult’s Law

It is a law of thermodynamics, explained by a French chemist François-Marie Raoult in 1887. Raoult’s law states that for a solution of volatile liquids, the partial vapour pressure of each component in the solution is directly proportional to its mole fraction. It is a chemical law, states that the vapour pressure of a solution relies on the mole fraction of a solute added to the solution. Vapour pressure is employed to estimate the contribution of individual components of a given liquid or solid mixture to the total pressure exerted by the system. This is applicable more commonly for discrete mixtures with the known quantity of each component. It is  expressed by the formula:

Psolution = ΧsolventP0solvent.

P1= p01x1: p02x2

p01x1= p02x2



Dalton’s law of partial pressure

Dalton’s law of partial pressure states that the total pressure over the solution phase in the container is the sum of the partial pressures of the components of the solution.  While the partial pressure of an individual gas is equal to the total pressure multiplied by the mole fraction of that gas. It is  used to determine the individual pressures of each gas in a mixture of gasses and is mathematically represented as:

Ptotal= p1+p2

When we substitute the corresponding values of p1 and p2 by p01x1and p02x2 respectively, we obtain

Ptotal= p01x1+ p02x2

= (1-x2) p01+ p02x2

= p01 +( p02- p01) x2



Mole fraction in vapor phase

The mole fraction of a given mixture in a particular phase irrespective of whether it is in the vapour or the liquid,  is the number of moles of that component in that phase divided by its total number of moles of each component in that phase. Binary mixtures are those having two components. In case h1 and h2 are the mole fractions of the components 1 and 2 respectively in the vapour phase, By using Dalton’s law of partial pressures, it is expressed as :

 P1=h1 ptotal

P2=h2 ptotal

The two equations can be represented by a general equation of the form: pi =hi ptotal



Vapor pressure of solutions of solids in liquids and Raoult’s law

Let us consider non-volatile solutes. When you add a non-volatile solute to a solvent the resultant solution has the reduced vapor pressure due to reduction in the number of solvent molecules which escape from the surface. The vapor pressure of a solvent in any case depends on the quality of non-volatile solute that is present n the solution irrespective of their nature. For the case of a binary solution, only the solvent molecules that are available in the vapor phase will lead to vapor pressure. For instance, if we let p1 to represent the vapour pressure of the solvent 1, and x1 represent its mole fraction and p01 be the vapor pressure of the solvent in its pure state. From Raoult’s law, it is true to say that the vapor pressure of this solvent is directly proportional to its mole fraction.


p1α x1

And p1=x1 p10=ptotal



Gases always tend to be less soluble in liquids as the temperature is raised because, when gases are dissolved in water, it is accompanied by the release of heat energy.  When the temperature is raised, according to Lechatlier’s principle, the equilibrium shifts in the backward direction, and becomes less soluble in liquids.





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