Advertisements
Advertisements
Question
Why is the mass determined by measuring a colligative property in case of some solutes abnormal ? Discuss it with the help of Van’t Hoff factor.
Advertisements
Solution
Certain solutes/compounds when dissolved in suitable solvents either dissociate or associate.
For example: (i) Ethanoic acid dimerises in benzene due to hydrofen bonding, while
(ii) In water, it dissociates and forms ions.
As a result, the number of chemical species in solution incerases or decreases as compared to the number of chemical species of solute added to form the solution. Since the magnitude of colligative property depends on the number of solute particles relative to the total number of particles in solution, it is expected that the molar mass determined on the basis of colligative properties will be either higher or lower than the expected value or the bormal value and is called abnormal molar mass.
In order to account for the extent of dissociation or association of molecules in solution, Van't Hoff introduced a factor, i know as the van't Hoff factor. It can be defined as follows.
i = `"Expected molar mass"/"Abnormal molar mass"`
= `"Observed colligative property"/"Calculated colligative property"`
= `"Total number of moles of particles after assocaition/dissociation"/"Number of moles of particles before assciation/dissociation"`
Here abnormal molar mass is the experimentally determined molar mass and calculated colligative properties are obtained by assuming that the non-volative solute is neither associated nor dissociated in case of association the value of i(van't Hoff factor) is lesser than unity while for dissociation iti is greater than unity.
APPEARS IN
RELATED QUESTIONS
The vapour pressure of pure benzene is 640mm og Hg. 2.175×10-3kg of non-vloatile solute is added to 39 gram of benzene the vapour pressure of solution is 600mm of HG. Calculate molar mass of solute.
[C = 12, H = 1]
The determination of molar mass from elevation in boiling point is called as
- cryoscopy
- colorimetry
- ebullioscopy
- spectroscopy
A solution of glucose in water is labelled as 10% (W/W).
Calculate:
a. Molality
b. Molarity of the solution.
[Given: Density of solution is 1.20 g mL-1 and molar mass of glucose is 180 g mol-1 ]
A solution of glucose (molar mass = 180 g mol–1) in water is labelled as 10% (by mass). What would be the molality and molarity of the solution ? (Density of solution = 1.2 g mL–1)
A solution of a substance having mass 1.8 x 10-3 kg has the osmotic pressure of 0.52 atm at 280 K. Calculate the molar mass of the substance used.
[Volume = 1 dm3, R = 8.314 J K-1 mol-1]
An organic substance (M = 169 gram mol–1) is dissolved in 2000 cm3 of water. Its osmotic pressure at 12°C was found to be 0.54 atm. If R = 0.0821 L atm K–1 mol–1, calculate the mass of the solute.
Write one difference in Coagulation and Peptization
Out of 0.1 molal aqueous solution of glucose and 0.1 molal aqueous solution of KCl, which one will have higher boiling point and why?
A solution 0.1 M of Na2SO4 is dissolved to the extent of 95%. What would be its osmotic pressure at 027 ? C (R = 0.0821 L atm K-1 mol-1) .
Define the following terms :
a. Cryoscopic constant
b. Resistivity
4.0 grams of NaOH ( Molar mass = 40.0 g mol-1 ) is dissolved in 500 cm3 of water. What is the molarity of NaOH solution?
Colligative properties depend on ______.
Which of the following is a colligative property?
Elevation in boiling point for 1.5 molal solution of glucose in water is 4 K The depression in freezing point of 4.5 molal solution of glucose in water is 4 K. The ratio of molal elevation constant to molal depression constant (Kb/Kf) is ______.
The depression in freezing point of water observed for the same amount of acetic acid, trichloroacetic acid and trifluoroacetic acid increases in the order given above. Explain briefly.
