Raoult’s Law

 Vapour pressures of solutions of solids in liquids and Raoult’s Law

A law stating that the freezing and boiling points of an ideal solution are respectively depressed and elevated relative to that of the pure solvent by an amount proportional to the mole fraction of solute.

• a law stating that the vapour pressure of an ideal solution is proportional to the mole fraction of solvent.

(Raoult’s law for non volatile solutes)

Q-1 what happens when a non volatile solute is added to a solvent?

1.  If a non-volatile solute is added to a solvent to give a solution, the number of solvent molecules escaping from the surface is correspondingly reduced, thus, the vapour pressure is also reduced.

Q-2 Which factor effects the vapour pressure of solvent ?

1.  The decrease in the vapour pressure of solvent depends on the quantity of non-volatile solute present in the solution, irrespective of its nature.

Q-3 What does Raoult law in its general form can be stated?  

1.  Raoult’s law in its general form can be stated as, for any solution the partial vapour pressure of each volatile component in the solution is directly proportional to its mole fraction.

Q-4 How to Denote binary solution?

1.  In a binary solution, let us denote the solvent by 1 and solute by 2.

Q-5 What happens when the solute is non-volatile? 

1. When the solute is non-volatile, only the solvent molecules are present in vapour phase and contribute to vapour pressure.

Q-6 How to use  Raoult's law?

1.  Let p₁ be the vapour pressure of the solvent, x₁ be its mole fraction, p⁰₁ be its vapour pressure in the pure state. Then according to Raoult’s law
2. p₁ 𝞪 x₁ and p₁ = x₁  p⁰₁ = p_(total)

If a solution obeys Raoult’s law for all concentrations, 

its vapour pressure would vary linearly from zero to the vapour pressure of the pure solvent.

According to Raoults law, for any volatile component of the solution. Pa=P°a×Xa. Hence vapour pressure is directly proportional to the mole fraction of solute.

If gas is a solute and liquid is the solvent, then according to Henry law, Pa=KaXa. Hence partial pressure of volatile component is directly proportional to mole fractionof that component.

Hence both are identical with only different proportionality constant's.

1. Raoult's law  is a law of thermodynamics established by French chemist François-Marie Raoult in 1887. It states that the partial vapor pressure of each component of an ideal mixture of liquids is equal to the vapour pressure of the pure component multiplied by its mole fraction in the mixture.
2. Henry's law is one of the gas laws formulated by William Henry in 1803 and states: "At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid."
3. Henry's law is one of the gas laws, formulated by the British chemist, WilliamHenry, in 1803. It states that: At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid

What is the relationship between Henry's law and Raoult's law?

Both are ideal vapor-liquid equilibrium relationships that hold for very dilute binary component mixtures.
  Henry's Law tends to approximate real behavior for the solute (i.e., lower concentration) at a very low concentration while Raoult's Law tends to approximate real behavior for the solvent in a dilute solution.
  They are both limit cases of an underlying binary vapor-liquid equilibrium relationship. 
 Unifying them basically requires that you replace the pressure terms with fugacity terms to accurately represent real gas behavior.

1. If you're familiar with chemical equilibria, it might be useful to think of the Henry's law 'constant' (not really a constant as it is dependent on temperature and ionic strength of the solution) as the equilibrium 'constant' of the reaction X(g) <-> X(aq), where X is the gas in question and is dissolving in water. (http://www.chemguide.co.uk/physi...) I.e. Henry' law constant is is the ratio of aqueous concentration to gas phase concentration of the gas AT EQUILIBRIUM.
2.  Any deviation from this, means the gas and liquid phases are not at equilibrium with respect to the particular gas and there will be a net transfer of gas molecules one way or another to move towards equilibrium (Le Chatelier's principle).
3.  Henry's law only holds for dilute solutions in single solvents (normally water).
4.  Henry's LAW basically says that the position of this equilibrium, i.e. the value of the equilibrium constant is always the same under the same conditions.
5. 
   
6. Raoult's law, as stated by one of the other answers, concerns the bulk components of a solution made up of different solvents, and their relative contributions to the total vapour pressure. In short, the total vapour pressure is the sum of the partial pressures:
7. 
   
8. vapour pressure of mixture = partial pressure of component A + partial pressure of component B +....
9. 
   
10. and each partial pressure is the product of the vapour pressure of a pure solution of the compound and the mole fraction of the compound in the solvent mixture (i.e. what proportion, by moles rather than mass, of the total mixture is made up of each particular compound):
11. 
    
12. partial pressure of A = vapour pressure of pure solution of A x mole fraction of A in mixture.
13. 
    
14. Both Henry's law and Raoult's make important assumptions about ideality of gases and solutions respectively.
15. 
    

Both laws can get into quite details and more complicated calculation, my answer just provide the base of both law. 

1. With a little more detail: 
2. Henry's law states that the solubility of a gas in a liquid is proportional to the pressure of the gas over the solution.
3. The equation is c = kP ,
4.  where c is the molar concentration in mol/L of the dissolved gas,
5.  P is the pressure (in atm) of the gas over the solution at equilibrium and k is a constant that depends only on temperature for a given gas.
6. Which means that when you want to mix a gas and a liquid,
7.  the amount of gas that will actually dissolve in the liquid is proportional to two things:
8.  the pressure of that gas at equilibrium over the solution and a constant k that changes depending on the gas and the temperature.
9.  By using the equation up there, you will find the molar concentration (mol/L) of the gas that will dissolve in the solution. 

On the other hand,

Raoult's law is used in a case where the solute (the smallest component of the solution) is non-volatile. The law considers that the vapor pressure of the whole solution will always be less than that of the pure solvent. Therefore, the vapor pressure of the solution will depends on the concentration of the solute. 

Raoult's law equation is P1 = X1 P1* (they are multiplied), where P1 is the vapor pressure of the solvent over the solution, X1 is the mole faction of the solvent in the solution and P1* is the vapor pressure of the pure solvent (if it was alone in the solution). 

Overall, the difference is that Henry's law takes care of what happen IN the solution when you have gas over it, while Raoult's law looks at what is happening OVER the solution when you mix a non-volatile solute to a solvent that has a known vapor pressure when it's pure (e.g. water). 
Henry's law will give you the molar concentration of a dissolved gas in the solution, Raoult's law will give you a vapor pressure over a solution after you mixed a solvent with a non-volatile solute. 
Hope it helped!

Raoult’s law explains partial vapour pressure of volatile component of the solution.

According to Raoult’s law,partial vapour pressure of volatile component is equal to product of vapour pressure of pure component and mole fraction of that component in the solution.

P =P⁰ X

Henry’s law explains the solubility of gas in liquid.

According to Henry’s law,pressure is directly proportional to the mole fraction of gas.

P = KH X

When KH and P⁰ are same ,then Raoult’s law becomes special case of Henry’s law. This happens in case of few ideal solutions of gas in liquid.

On the other hand,

1. Raoult's  law is used in a case where the solute (the smallest component of the  solution) is non-volatile.
2. The law considers that the vapor pressure of  the whole solution will always be less than that of the pure solvent.
3. Therefore, the vapor pressure of the solution will depends on the  concentration of the solute. 
4. 
   
5. Raoult's law equation is P1 = X1  P1* (they are multiplied),
6.  where P1 is the vapor pressure of the solvent  over the solution,
7.  X1 is the mole faction of the solvent in the  solution
8.   P1* is the vapor pressure of the pure solvent (if it was  alone in the solution). 
9. 
   
10. Overall, the difference is that Henry's  law takes care of what happen IN the solution when you have gas over it, 
11.  while Raoult's law looks at what is happening OVER the solution when  you mix a non-volatile solute to a solvent that has a known vapor  pressure when it's pure (e.g. water). 
12. Henry's law will give you the  molar concentration of a dissolved gas in the solution,
13.  Raoult's law  will give you a vapor pressure over a solution after you mixed a solvent  with a non-volatile solute.