# Brief information of Rault’s law in chemistry

- In today’s article, we are going to study about ‘ Brief information of Rault’s law in chemistry’ and what is its essential condition. How beneficial it is and what is the difference between positive and negative deviation of a solution. We will also read about some more topics like expression for Raoul’s law, positive and negative deviation, expression for positive deviation and negative deviation, phase diagram etc in detail. So, take your notebooks in you hand and get ready to study physics in an easy and sorted way.

## Rault’s Law

The vapour pressure of a liquid is the measurement of escaping tendency of its molecules in vapour state. If we have a solution containing volatile components A and B. Then molecules of each component have their own tendency of escaping which is called its partial vapour pressure in the solution.

If PA and PB are the partial vapour pressures of A and B respectively and p be the total vapour pressure of the solution. Then According to Rault’s Law ” **At a given temprature the vapour pressure of any volatile components of a solution is equal to the vapour pressure of the pure components multiplied by the mole fraction of that component in solution.” **An ideal solution obeys Rault’s Law over the whole range of concentration.

Let us consider a liquid pair A and B which are volatile, completely miscible and form an ideal solution. Suppose XA and XB be the mole fractions of A and B in the solution and PA^° and PB^° are the vapour pressure of liquids A and B in pure state, then according to Rault’s Law.

pA= pA^° XA …(3)

pB= pB^° XB …(4)

If p is the vapour of solution, then according to Dalton’s law of partial pressure.

p= pA + pB …(5)

p= pA^° XA + pB^° XB

p= pA^° (1- XB) + pB^° XB …(6)

(Since XA and XB = 1)

p= PA^° + XB (PB^° – PA^°) …(7)

Equation (3), (4) and (5) represents equation of straight line. If a graph is plotted between vapour pressure and mole fractions it is obtained according to fig.

In fig dotted lines are obtained from equations (3) and (4) which represents partial vapour pressures of A and B. When XA =1, i.e. only pure component A is present, p=pA^°. Whereas, if XA=0 i.e. pure component B is present, pB=pB^°. The solid line indicates the total vapour pressure of the ideal solution. At any concentration the total pressure is equal to the sum of the partial pressures of two components.

Few examples of ideal solution are:

- n-Hexane and n-heptane
- Ethyl bromide and ethyl iodide
- Benzene and toluene
- Cholorobenzene and Bromobenzene

### Deviation from Rault’s Law (Non- ideal solution)

As stated earlier cohesive forces are uniform in ideal solutions. If cohesive forces between solute- solvent are different tham those of between solute-solute and solvent -solvent is called non-ideal.

Suppose a solution is made of two components A and B. If interaction between A-A and B-B is different than interaction between A-B, the solution will behave like a non ideal solution.

#### Positive deviation from Rault’s Law

If the cohesive forces between A-B are weaker than those of the pure liquids (A-A and B-B) this leads to the positive derivation from ideality. For such solution.

PA > pA^°XA

And PB> pB^°XB

In such solutions the escaping tendencies of the components in the solution are higher than the escaping tendencies of the individual pure components. Thus, vapour pressure of solution increases.

Few solution showing positive deviation are:

- Ethanol and water
- Ether and acetone
- Carbon disulphide and acetone
- Carbon tetrachloride and methanol
- Carbon tetrachloride and ethanol
- Cyclohexane and ethanol
- Cyclohexane and methanol
- Carbon tetrachloride and toulene.

Vapour pressure curve of ethanol-water System is shown in fig.

Dotted line exhibit the vapour pressure of ideal solution whereas dark line shows the vapour pressure of non-ideal solution in the fig.

##### For a position deviation solutions:

- ΔHmix > (endothermic process)
- ΔHmix > 0 (volume of solution is more than the sum of volumes of components)

### Negative deviation from Rault’s Law

If the cohesive forces between A-B stronger than those of the pure liquids (A-A and B-B) this leads to the negative deviation from ideality. For such solutions

PA < pA^°XA

And PB< pB^°XB

In such solutions the escaping tendency of component from the solution would be less than it would be from the pure liquid. Thus, vapour pressure of solution decreases.

Few examples of liquid pairs which shows negative deviations from ideality are:

- Chloroform- Acetone
- Methanol- acetone
- Pyridine- acetic acid
- Water – nitric acid
- Acetone- Aniline

Fig shows the vapour pressure curve for a negative deviation solution.

Dotted line shows the behaviour of ideal solutions whereas dark line exhibit the behaviour of non ideal solution.

##### For a negative deviation solution :

- ΔHmix < (exothermic process)
- ΔHmix > 0 (volume of solution is less than the sum of volumes of components)

#### Conclusion

So our article is finished and after completely reading this article, one can easily tell what Rault’s law and what are it’s types.

We can also tell actual difference between positive and negative deviation. And we covered some other topics also, such as phase diagram of Rault’s Law, expression for positive and negative deviation etc.

So one can say that they got a detailed information about Rault’s law.

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