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Osmotic Pressure

Which colliga...

Question

Which colligative property is more useful to determine the molecular weight of the substances like proteins and polymers ?

A

Elevation in boiling point

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B

Lowering of vapour pressure

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C

Depression of freezing point

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D

Osmotic pressure

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Solution

The correct option is B Osmotic pressure
Osmotic pressure is widely used to determine molar masses of proteins, polymers and other macromolecules.
$M_{2} = \frac{w_{2} R T}{π V}$
where, $π$ = osmotic pressure, R gas constant,
V = volume of the solution (in L), $M_{2}$ = molar mass, $w_{2}$ = weight of solute

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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. Which of the following colligative property is useful to determine molecular masses of polymer compounds from their solutions?

Q. The best colligative property used for the determination of molecular masses of polymers is:

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.

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:

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