Lab Report : Colligative properties of solution

Sambeg Raj Subedi

The City College of New York

 Abstract

The main objective of this experiment is to demonstrate the movement of solvent through a membrane as a result of osmotic pressure, to measure the depression in freezing point for water resulting from the addition of solute and evaluate its freezing point constant and to measure the elevation in boiling point for water resulting from the addition of a solute and evaluate its boiling point constant. In this experiment three sets of test will be performed to show the colligative properties of solution.

Key-words: Osmosis, Freezing point Depression, Boiling point elevation

 

Introduction :

            When a solute is dissolved in the solvent to form the solution then certain properties of solvents changes such as freezing point, boiling point, vapor pressure, and osmotic pressure. Such properties are called Colligative properties. For Example, During the snowy days, we can see that salt is spread on the road. If we closely analyze this event then we can figure out some connection in between salt and ice. When salt is spread on the snowy or icy road then freezing point of that salt-water is less than that of pure water. As a result, there is less chance of ice formation which will certainly help in keeping the road safe for commuters.

Osmosis and Osmotic pressure:

When two solutions of different concentration are separated by semipermeable membrane then solvent from lower concentration moves towards a solution with higher concentration such phenomenon is called Osmosis and pressure required to prevent solvent flow is called osmotic pressure. In simpler words, “Osmosis generally refers to the net movement of water across a membrane. Osmosis occurs, for example, when an aqueous solution is separated from pure water by a semi-permeable membrane”(Kowles,2010).

Freezing Point Depression:

It is the decrease of the freezing point of the solvent on an addition of the non-volatile solute. The concept of freezing point depression is necessary to understand because it helps us to know about the formation of ice in the atmosphere. Cirrus cloud, which looks like a thin wispy streamer in the sky, mostly seen during fair weather is actually an ice crystal which is optically thin in the visible wavelength range. They allow solar radiation to reach the earth surface. Similarly, they also contribute to global warming because they are strong infrared absorber. Thus, determining the temperature at which Cirrus cloud will be formed is important to understand their effect on global radiation(Bodzewski,2016). This can be considered as an application of Freezing point Depression. The difference between the temperature of pure solvent and solution freezing point is ;

where, m is molal concentration of solute in solution

k_f is freezing point constant of solvent.

Boiling point elevation:

It is the increase in the boiling point of the solvent on addition of the solute.

The relationship between  Boiling point of a solution compared to the pure solvent is  expressed by,

Where,

m is molal concentration of solute in solution

k_b is Boiling point constant of solvent.

Material Required:

Beaker, Graduated cylinder, Electronic Balance, Thermometer, Test-tube holder, Stirring rod, Stand, Clamp, Ice cubes

Chemical Required:

Distilled water, Sugar solution, Sodium chloride, Ethylene glycol, Sucrose

Procedure for Osmosis and Osmotic Pressure

Initially, About 10g of sucrose was mixed in the 90ml of distilled water by swirling 125ml flask. Then, two 8cm lengths of dialysis tube was obtained and moisten them in water where one of the ends in both dialysis tube was tied with string tightly to form a tube. 5ml of sugar solution was kept in one of dialysis bag and 5ml of distilled water was kept in another bag making sure that it didn’t leak from the bottom. Both of the tubes were rinsed, dried and their weight was recorded in the lab sheet. After that, the remaining sugar solution was kept inside 250 ml beaker and 100ml of distilled water was kept into another 250 ml beaker which was labeled as A and B. Then, the dialysis bag with distilled water was transferred to beaker with sugar solution and dialysis bag with sugar solution was transferred to the beaker with distilled water. Along with that, two dried prunes were obtained and each of their mass was recorded. Then, each of the prunes was kept inside both of the beakers. Both of the beakers was left for an hour for the result and after an hour both of the dialysis bag and prune were taken out, rinsed and dried as before. After that process, their weight was taken and recorded.

 

Procedure for freezing point depression

Initially, the clean, dry and empty 100ml beaker was taken and ½ fully crushed ice was added to it.  About 25 ml of distilled water was measured by using a graduated cylinder, poured it into the beaker with an ice and their weight was recorded as shown in a balance. Then ice and water were stirred together with the glass rod. After stirring, the temperature was measured with a thermometer until the temperature was nearest to 0.1℃ which was the freezing point of the pure water. About 5g NaCl was added to the mixture where the temperature of the mixture was measured again after stirring with the glass rod until the freezing point was obtained approximately about 5-8 minutes. This process was repeated 1-4 times by using 50 ml beaker. 4.5ml of Ethylene glycol was put into the beaker and the combined mass of the mixture was measured in the balance.

The mixture was mixed with the glass rod to get the freezing point of the solution.

 

Procedure for Boiling point elevation

Initially, 50ml of distilled water was placed in the 100ml of beaker where 1 or 2 boiling chips were added to it. Then the beaker was kept in a hotter to heat until the water inside it boils steadily. After that, the temperature of boiling water was measured with the help of thermometer to obtain the boiling point of the water. 20g of sucrose was measured in the balance and added to the water in the beaker. Then, the mixture was stirred carefully with a glass rod and the temperature was measured as soon as the mixture started to boil steadily. This process was repeated 1-7 times by substituting.

Data and Calculation:

1. Osmosis and Osmotic Pressure

 

Table 1:  Samples placed in sugar solution

 

Sample	Mass before soaking	Mass after soaking
Dialysis tubing containing H2O	6.05 g	5.62 g

 

Table 2 – Samples placed in water solution

 

Sample	Mass before soaking	Mass after soaking
Dialysis tubing containing sugar solution	5.33 g	5.44g

 

Table 3: REPORT

 

Sample	Surrounding liquid	Change in sample mass	Direction of solvent flow
Dialysis tubing + water	Sugar solution	0.43	+
Dialysis tubing + sugar solution	Pure water	0.10	+

 

 

2. Freezing-Point Depression

 

Calculation for Freezing point Depression :(From experiment)

 

• Moles of NaCl =  mass NaCl    x   1 mol NaCl/ 58.5 g

=   5 g       x   1 mol / 58.5 g

=   0.086 mol

 

• Moles of C₂H₆O₂=  mass C₂H₆O₂    x   1 mol C₂H₆O₂ / 62.0 g

=   5.35g           x   1 mol / 62.0 g

=   0.086 mol

 

• ⧍t_f  NaCl    =   freezing point of water – freezing point of NaCl

=   1 ^OC   –   (-2^OC)

=   3^OC

 

• ⧍t_f  C₂H₆O₂      =   the freezing point of water – the freezing point of C₂H₆O₂

=   1 ^OC   –  (-2 ^OC)

=   3 ^OC

 

• Molality           =   moles of solute / kg of solvent

 

Molality NaCl    =   0.086 mol / 0.052 kg

=   1.62 mol/kg

           

• Molality C₂H₆O₂=   0.086 mol / 0.058 kg

=   1.483 mol/kg

 We know,

 

 

• k_f  NaCl    =  3^OC/ 65 mol/kg

= 1.744 ^OC kg/mol

 

• k_f  C₂H₆O₂=  3^OC / 43mol/kg

= 2.02^OC kg/mol

 

3. Boiling-Point Elevation

Calculation for Boiling point elevation : (From experiment)

 

• Moles of C₁₂H₂₂O₁₁=  mass C₁₂H₂₂O₁₁    x   1 mol C₁₂H₂₂O₁₁/ 58.5 g

=   20 g             x   1 mol / 58.5 g

=   0.058 mol

 

• Moles of C₂H₆O₂=  mass C₂H₆O₂    x   1 mol C₂H₆O₂ / 62.0 g

=   19.50g           x   1 mol / 62.0 g

=   0.314 mol

 

• ⧍t _b C₁₂H₂₂O₁₁     =   Boiling point of C₁₂H₂₂O₁₁  –  Boiling point of water

=   102.00 ^OC   –   99.6 ^OC

=   2.4 ^OC

 

• ⧍t_b  C₂H₆O₂      =   Boiling point of C₂H₆O₂ –  Boiling point of water

=   105 ^OC   –   99.6^OC

=   5.4 ^OC

 

• Molality   =   moles of solute / kg of solvent
• Molality C₁₂H₂₂O₁₁=   0.058 mol / 0.05 kg

=   1.16mol/kg

• Molality C₂H₆O₂ =   0.314 mol / 0.05 kg

=   6.28 mol/kg

 

We know,

 

 

• k_b C₁₂H₂₂O₁₁=  2.4 ^OC/ 16mol/kg

= 2.06^OC kg/mol

 

• K_b C₂H₆O₂= 5.4^OC/ 28 mol/kg

= 0.85^OC kg/mol

Discussion and Result:

This experiment involved the three sets of test, the first test was about osmosis, as we know, if two solutions of different concentration are separated by a semipermeable membrane through which solvent can flow, the solvent will flow from a dilute solution into the concentrated solution. So, as expected when dialysis tube containing water was placed in the sugar solution for almost an hour, the mass of tube was found to be less than previous because solvent i.e water moved from tube to the sugar solution which has a higher concentration. When dialysis tube containing sugar solution was kept in water, we also obtained an expected result, where the mass of tube filled with sugar solution after soaking was more than before soaking. This is a successful experiment because water should always move towards sugar solution resulting in higher mass after soaking. There could be several errors while performing this experiment. Possible error in this experiment might be improper handling of the tubes, leakage in the tube which might decrease the final mass and also the incorrect reading of the measurement.

In the second test, through the series of experimental steps or procedure, we measured the freezing point depression for water with respect to NaCl solute and Ethylene Glycol solute and calculated freezing point constant as 1.744 OC kg/mol and 2.02 OC kg/mol respectively which is close to the theoretical value 1.86 OC kg/mol. Initially, while performing the experiment, we noted 1 OC as 0.1 OC and -2 OC  as -0.2 OC  which was later realized and correction was made.

The final test was about determining the Boiling Point Elevation, here we measured the elevation in boiling point for water resulted due to addition of sucrose and Ethylene Glycol individually and obtained the Boiling point constant as 2.069 OC kg/mol and 0.859 OC kg/mol respectively, but the theoretical value is 0.51 OC kg/mol. Some difference in the result might have occurred due to an experimental error such as an inability to determine the exact boiling point of the sucrose solute-solution.

Error:

This experiment was highly sensitive because it requires accuracy in measurement and proper handling of the materials. For example, in the first test related to osmosis, dialysis tube containing either water or sugar solution should be properly measured and prevent it from leaking. Unable to do so may greatly impact on the result. Secondly, measuring the temperature in the thermometer. It should be done accurately because it plays a major role in obtaining an accurate result. And lastly, making the proper measurement. These were the possible error that might affect the calculation and in-order to minimize the error, an experiment should be performed carefully and special attention should be given while measuring the temperature in the thermometer.

Conclusion:

In this experiment, though there occurred some experimental errors, evaluating those error too, it was observed that solvent from lower concentration solution flow towards higher concentration through a semipermeable membrane and flow of the solvent through a membrane as a result of osmotic pressure. The freezing point constant was 1.744 ^OC kg/mol for sodium chloride solute and 2.02 ^OC kg/mol for Ethylene Glycol solute. Similarly, the boiling point constant was obtained 2.069 ^OC kg/mol for sucrose solute and 0.859 ^OC kg/mol with respect to Ethylene glycol.

 

 

REFERENCES:

 

Kowles, R. V. (2010). Regulation of Water in Plant Cells. Bioscene: Journal of College Biology Teaching, 36(1), 34–42.

Bodzewski, K. Y., Caylor, R. L., Comstock, A. M., Hadley, A. T., Imholt, F. M., Kirwan, K. D., … Wise, M. E. (2016). Investigating Freezing Point Depression and Cirrus Cloud Nucleation Mechanisms Using a Differential Scanning Calorimeter. Journal of Chemical Education, 93(4), 729–732.

Petrucci (Ed.)(2012); SCC202 Fundamentals of Chemistry II Laboratory Manual; Cengage Learning, pp 21 – 26.