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Relative Lowering of Vapour Pressure

Among the fol...

Question

Among the following which is not a colligative property?

A

Osmotic pressure

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B

Elevation of boiling point

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C

Vapour pressure

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D

Depression of freezing point

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Solution

The correct option is D Vapour pressure
The properties of the solutions which depends on the number of particles of the solute but not on the mass of the solute in the solvent are called colligative properties. Examples:
1) Elevation in boiling point
2) Depression in freezing point
3) Depression in vapour pressure
4) Osmatic pressure

Hence option A is correct.

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Q. The experimental values of colligative properties of many solutes in solution resembles calculated values of colligative properties.
However in some cases, the experimental values of colligative properties differ widely than those obtained by calculations. Such experimental values of colligative properties are known as abnormal values of colligative properties. The causes for abnormal values of colligative properties are:
(i) Dissociation of solute: It increases the colligative properties.
e.g.: Dissociation of

KCl, NaCl

H_{2} O

.
(ii) Association of solute: It decreases the colligative properties.
e.g.: Dimerisation of acetic acid in benzene.

If the degree of dissociation of an electrolyte

A_{2} B_{3}

is 25% in a solvent, then:

Properties such as boiling point, freezing point and vapour pressure of a pure solvent change when solute molecules are added to get homogeneous solution. These are called colligative properties. Applications of colligative properties are very useful in day-to – day life.

One of its examples is the use of ethylene glycol and water mixture as anti-freezing liquid in the radiator of automobiles.

A solution M is prepared by mixing ethanol and water. The mole fraction of ethanol in the mixture is 0.9.

Given, freezing point depression constant of water

$(K_{f}^{w a t e r}) = 1.86 K k g m o l^{- 1}$

Freezing point depression constant of ethanol

$(K_{b}^{w a t e r}) = 2.0 K k g m o l^{- 1}$

Boiling point elevation constant of water

$(K_{b}^{w a t e r}) = 0.52 K k g m o l^{- 1}$

Boiling point elevation constant of ethanol

$(K_{b}^{e t h a n o l}) = 1.2 K k g m o l^{- 1}$

Standard freezing point of water = 273 K

Standard freezing point of ethanol = 155.7 K

Standard boiling point of water = 373 K

Standard boiling point of ethanol = 351.5K

Vapour pressure of pure water = 32.8 mm Hg

Vapour pressure of pure ethanol = 40 mm Hg

Molecular weight of water $= 18 g m o l^{- 1}$

Molecular weight of ethanol $= 46 g m o l^{- 1}$

In answering the following questions, consider the solutions to be ideal dilute solutions and solutes to be non-volatile and non-dissociative.

The vapour pressure of the solution M is

Q. Read the passage given below and answer the following questions:
Read the passage given below and answer the following questions:
A soluble solid in solution has the effect of raising its boiling point and depressing its freezing point. The addition of non-volatile substances to a solvent decreases the vapor pressure and the added solute particles affects the formation of pure solvent crystals. According to many researches the decrease in freezing point directly correlated to the concentration of solutes dissolved in the solvent. This phenomenon is expressed as freezing point depression and it is useful for several applications such as freeze concentration of liquid food and to find the molar mass of an unknown solute in the solution. Freeze concentration is a high quality liquid food concentration method where water is removed by forming ice crystals. This is done by cooling the liquid food below the freezing point of the solution. The freezing point depression is referred as a colligative property and it is proportional to the molar concentration of the solution (m), along with vapor pressure lowering, boiling point elevation, and osmotic pressure. These are physical characteristics of solutions that depend only on the identity of the solvent and the concentration of the solute. The characters are not depending on the solute's identity.

Identify which of the following is colligative property:

Q. The experimental values of colligative properties of many solutes in solution resembles calculated values of colligative properties.
However in some cases, the experimental values of colligative properties differ widely than those obtained by calculations. Such experimental values of colligative properties are known as abnormal values of colligative properties. The causes for abnormal values of colligative properties are:
(i) Dissociation of solute: It increases the colligative properties.
e.g.: Dissociation of

KCl, NaCl

H_{2} O

.
(ii) Association of solute: It decreases the colligative properties.
e.g.: Dimerisation of acetic acid in benzene.

One mole of

I_{2}

(solid. is added in

1 M

KI

solution. Then,

Q. Consider the following statements and arrange in the order of true/false as given in the codes.

S_{1}

: Vapour pressure is a colligative property.

S_{2}

: Freezing point of a solution is always lower than that of the pure solvent.

S_{3}

: Acetic acid undergoes association in benzene. The molar mass of acetic acid, determined by elevation of boiling point is always higher than its normal molar mass.

S_{4}

: Osmotic pressure measurements can be used for determination of molar mass of polymers.

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