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Boiling Point Elevation Experiment

 

Lab Report on study of boiling points of solutions of various concentrations involving electrolytes and non electrolytes is as follows.


Introduction

The temperature at which the vapour pressure of the liquid is same as that the atmospheric pressure of liquid is called Boiling point. When a nonvolatile solute is added to a solvent the vapour pressure of the solution is decreased. Therefore, the boiling point of the solution is increased or elevated. Elevation in boiling point is a colligative property; ie, the boiling point depends on the number of solute particles. According to Raoult's law the elevation in boiling point is directly proportional to the molality of the solution. The relation between the elevation in boiling point and the molecular mass of the solute is given by


M = 1000 Kb WB/ ΔT WA


Where M is the molecular mass of solute, Kb the molal elevation constant of the solvent, WB the mass of the solute, WA is mass of the solvent and ΔT the elevation in boiling point.


Aim


To study the boiling points of solutions of various concentrations involving electrolytes and non—electrolytes


Requirements


Urea, NaCl, sensitive thermometer, boiling point apparatus etc.,


Procedure


Take 20gm of water in a boiling point apparatus. Fit it with a one holed cork through which a sensitive thermometer is introduced. It is clamped to an iron stand and is heated using a burner. The boiling point is noted. Now remove the thermometer and allow the liquid to cool in air and add 0.2g of urea in it and stir it with a glass rod. Introduce the thermometer in it. Heat the solution using a burner. Note the temperature at which the solution boils. Repeat the experiment by using 0.4g, 0.6g, and 0.8g of urea with the same quantity of water in each case. Note the boiling points of these solutions and tabulate. The above experiment is repeated by using 0.2g, 0.4g, 0.6g and 0.8g of sodium chloride in the same quantity of water. The boiling points of these solutions are also tabulated.


It is found that the boiling point of solution increases with increase in concentration of the solution. Compared to urea solution the boiling point of sodium chloride solution is found to be much higher.


Conclusion


Urea is a non electrolyte while sodium chloride is an electrolyte. The number of particles in a solution of an electrolyte is higher than that of a non electrolyte. Since elevation of boiling point is a colligative property, the elevation in boiling point of sodium chloride is higher than the expected value.


VIVA QUESTIONS WITH ANSWERS


1. What is meant by colligative property?


Ans: Property which depends upon the number of particles is called colligative property.


2. How is elevation in boiling point of a liquid related to the molecular mass of the solute dissolved in it?


Ans: Elevation in boiling point of a liquid is inversely proportional to the molecular mass of the solute dissolved in it.


3. Among equimolar solutions of urea and sodium chloride, which has higher boiling point?


Ans: Sodium chloride solution has higher boiling point


4. Why does sodium chloride solution have higher boiling point than glucose solution of the same concentration?


Ans: Sodium chloride being an ionic compound dissociates to give two particles per molecule. Since boiling point is a colligative property the boiling point of sodium chloride solution is higher than the expected value.


5. If you take equimolar solutions of glucose and urea which will have higher boiling point? Why?


Ans: They have the same boiling point. This is because, being covalent compounds, equimolar solutions of glucose and urea contain the same number of particles.


6. Name some examples of ionic compounds.


Ans: KCl, NH4Cl, K2SO4, Na2SO4 etc.

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