The boiling-point elevation can in principle be used to measure the degree of dissociation or the molar mass of the solute. This kind of measurement is called ebullioscopy .This property can also be used in separating zeotropic mixtures.
AIM: To study the effect of solid impurities on boiling point of water and find the Molal Elevation Constant (Kb) and to calculate the percentage difference from accepted value for Kb. APPARATUS: 3 boiling tubes each fitted with a cork with 2 holes, 1100C thermometers with 0.10 C calibrations, Tripod stand, Wire mesh, Bunsen burner, Iron stand, China dish, weighing balance.
Chemicals Required- Sodium chloride (NaCl), Potassium sulphate (K2SO4), Ferric chloride (FeCl3), Water.
THEORY:
Colligative Properties
Colligative properties are properties of a solution containing 2 or more components. Typically, the primary component is called the solvent and the other components are called the solutes. Unlike other physical properties such as color or hardness, colligative properties depend only on the number of particles of solute in the solution, not the identity of the particles.
In chemistry, colligative properties are properties of solutions that depend upon the ratio of the number of solute particles to the number of solvent molecules in a solution. They are independent of the nature of the solute particles, and are due essentially to the dilution of the solvent by the solute.
Colligative properties include:
(1) Relative lowering of vapor pressure;
(2) Elevation of boiling point;
(3) Depression of freezing point and
(4) Osmotic pressure
The van’t Hoff Factor
For inorganic salts in an aqueous solution, the salt will dissociate into cations and anions. If the salt fully dissociates, the total number of particles in the solution will be based on the addition of the number of moles of cations and anions produced from the dissociation. Thus;
NaCl → Na+ + Cl-
(2moles particles produced)
CaCl2→Ca2++ 2Cl-
(3moles particles produced)
The quantity of particles produced per mole of solute is called the van’t Hoff factor and is given the symbol, i
Boiling Point Elevation
When a solution is heated the vapour pressure of the solvent decreases in the presence of non-volatile solute (impurities). In order to make the solution boil, its vapour pressure must be increased by raising the temperature above the boiling temperature of the pure solvent. Thus the presence of a non-volatile solute causes the solvent to boil at a higher temperature and the boiling point of a solution is always higher than that of pure solvent
The difference between the boiling point of the pure liquid and the boiling point of the solution is ∆Tb = Tb (solution) – Tb (pure)
The magnitude of this boiling point elevation is directly proportional to the concentration of solute, expressed as the molality; m. Kb is called the Boiling Point Elevation Constant of the solvent and is a function of the solvent only. It is not dependent on identity of the solute. For water, the Kb is equal to 0.512°C/m. The van’t Hoff factor, i, is used to give a better representation of the number of particles in the solution by estimating the degree of dissociation. Overall, the relationship between the temperature change and the concentration is: ∆Tb = i*Kb*m.
PROCEDURE:
- Take a beaker and add 50 ml of water in it
- Heat the beaker on a Bunsen burner with a wire mesh and thermometer inserted in it till water starts boiling observing the temperature after every 10 seconds
- Record this observation as boiling point of water in the table
- Prepare a NaCl solution by mixing 5.5g of NaCl in 50 ml water in a beaker.
- When the NaCl gets dissolved in water heat the above solution on a Bunsen burner with a wire mesh and thermometer inserted in it till the solution starts boiling observing the temperature every 10 seconds
- Record this observation as boiling point of NaCl solution in the observation table
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Repeat the above steps for K2SO4 solution and FeCl3 solution of approximately same molality as the NaCl solution
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Record these observations as boiling point of K2SO4 solution and FeCl3 solution in the observation table
Observation:
The molality of solutes in the solutions is:
[(5.5/58.5)/50]*1000= 1.88 {NaCl}
[(16.5/174)/50]*1000 = 1.89 {K2SO4}
[(15.2/162.2)/50]*1000 = 1.87 {FeCl3}
Result:
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The maximum rise in boiling point is observed in FeCl3 solution (4.2C) followed by K2SO4 solution (3.1C) and then NaCl solution (1.9C). As the Van’t Hoff factor is maximum for FeCl3 (4) followed by K2SO4 (3) and then NaCl (2), our data proves that elevation in boiling point is a colligative property
- The calculated mean value of Molal Elevation Constant of water is 0.538C/molal. This is in accordance with the accepted value 0.512C/molal.
- The percentage deviation from accepted value is calculated as follows
(0.538-0.512/0.512)*100 = (0.026/0.512)*100 = 5.08%
Bibliography
- Chemistry Part I Textbook for Class XII by NCERT, 2007
- Core Laboratory Manual In Chemistry for Class XII, 2010
- www.wikipedia.org
- http://www.chm.uri.edu/sgeldart/chm102/102%20Exp%209.pdf
Index
1 Introduction 4
2 Aim 5
3 Apparatus 5
4 Theory 5-6
5 Procedure 7
6 Observations 8
7 Result 9
8 Bibliography 10