Miscible liquid pairs often show negative or positive deviation from Raoult’s law. What is the reason for such deviations? Give one example of each type of liquid pairs.

Answer:
When a solution does not obey Raoult’s law over the entire range of concentration, then it is called non-ideal solution.

The vapour pressure of such solution either be higher or lower. i.e positive when higher
negtive when lower.

The cause for these deviations lie in the nature of interactions at the molecular level.

In case of positive deviation from Raoult’s law,
A-B interactions are weaker than those between A-A or B-B, i.e., in this case the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules. This means that in such solutions, molecules of A (or B) will find it easier to escape than in pure state. This will increase the vapour  pressure and result in positive deviation. Mixtures of ethanol and acetone behave in this manner.

In case of negative deviations from Raoult’s law, the intermolecular attractive forces between A-A and B-B are weaker than those between
A-B and leads to decrease in vapour pressure resulting in negative deviations. An example of this type is a mixture of phenol and aniline.
In this case the intermolecular hydrogen bonding between phenolic proton and lone pair on nitrogen atom of aniline is stronger than the
respective intermolecular hydrogen bonding between similar molecules.



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State the type of non-ideality exhibited by a solution of cyclohexane and ethanol or a solution of acetone and chloroform (only one case). Give reason for your answer.

The solution of ethanol and cyclohexane show positive deviation.

This is because when pure ethanol is added to cyclohexane the molecule of cyclohexane come in between the molecule of ethanol which result in the breaking f hydrogen bond and thus positive deviation .

Another case can be, the mixture of chloroform and acetone shows negtive deviation. When these are mixed , hydrogen bond is formed and negative deviation shown;

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State with a suitable diagram and appropriate examples why some non-ideal solutions. Show positive deviation from ideal behaviour.


In case of negative deviations from Raoult's law, the intermolecular attractive forces between A-A and B-B are weaker than those between A-B and leads to decrease in vapour pressure resulting in negative deviations. An example of this type is a mixture of phenol and aniline.

In this case the intermolecular hydrogen bonding between phenolic proton and lone pair on nitrogen atom of aniline is stronger than the respective intermolecular hydrogen bonding between similar molecules.



This decreases the escaping tendency of molecules for each component and consequently the vapour pressure decreases resulting in negative deviation from Raoult's law

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State Raoult’s law for solutions where only solvent is volatile. Derive a mathematical exression for this law.


Answer:

Raoult's law states that for a solution of volatile liquid, the partial vapour pressure of each component in the solution is dirctly proportional to its mole fraction.

Thus if there is solution of two liquid component 1 and 2 then,

for component 1,

p1  x1Andp1 = p10 x1For component 2 ,p2  = p20 x2where p1 p2 = partial vapour pressure of two volatile component in solution1 and 2 of the solution p10 p20 = vapour pressure of pure component 1and 2x1 x2 = mole fraction of component

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What is meant by abnormal mass of solute? Discuss the factors which bring abnormality in the experimentally determined molecular masses of solutes using colligative properties.

Answer:

A molar mass that is either lower or higher than the expected or normal value is called as abnormal molar mass.

In order to account for all such abnormalities, introduced a factor
(i) known as van 't Hoff's factor, which represents the extent of association (or) dissociation of a solute.

Van't Hoff's factor is defined as the ratio of the observed colligative property to the calculated colligative property.

i = Observed colligative property / Calculated colligative property

Observed colligative property ∝ 1/Molar Mass

i = Mc/Mo

Van't Hoff's factor
(i) represents the extent of association (or) dissociation of a solute

i = Total number of moles of particles after association or dissociation / Number of moles of particles before association or disscussion

Experimentally determined molar mass is always lower than actual value for dissociation.

Molar Mass ∝ 1/Colligative Property

If the solute undergoes association in a solution, then the value of van 't Hoff's factor is less than one. If the solute undergoes dissociation then 'i' is greater than one.

Ex:

                       KCl → K+ + Cl-

     1 molecule of KCl furnishes 2 ions in solution

i = Total number of moles of particles after dissociation / Number of moles of particles before dissociation

 i = 2/1 = 2

           2CH3COOH ⇔(CH3COOH)2
          Ethanoic acid     Dimer of Ethanoic acid

i = Total number of moles of particles after association / Number of moles of particles before association

i = 1/2 =  0.5

 

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