Exp 4

Universiti Tunku Abdul Rahman (Kampar Campus) Faculty of Science, Engineering, and Technology Bachelor of Science (Hons) Biotechnology Year 1 Semester 2 Laboratory 1B (UESB 1212) (II) The Properties of Matter Lecturer: Ms. Chew Yin Hoon Student’s Name: Cheah Hong Leong Student’s ID: 08AIB03788 Partner’s Name: Chong Shi Fern Partner’s ID: 08AIB02580 Experiment No. 4 Title: Determination of Relative Molecular Mass by Endpoint Cryoscopy. Date: 27 February 2009

Title: Determination of Relative Molecular Mass by Endpoint Cryoscopy

Objective: –

Determine the freezing point of Dimethyl Sulfoxide (DMSO).

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Determine the freezing point depression constant when naphthalene dissolved in DMSO.

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Determine the relative molecular mass of an unknown liquid from the freezing depression constant.

Theory and Background: Solution is the special type of mixture where the mixing between the solvent and solute are only at molecular level. The formation of solution only possible when the free energy of the solution is favored, that means the free energy of solution is lower than that of the pure solvent. Freezing point depression is one of the colligative properties of solution, meaning that this property is universal to all solutes in all solution regardless the identities of the solvent and solute. It also regardless whether the solute is electrolyte or nonelectrolyte. The freezing point depression is only dependent on the presence of solute in solvent and their quantities. When there is solute dissolved in a solvent forming a solution, the freezing point of the solution is lower than the pure solvent due to the presence of added solute. Freezing point depression in a solution is due to the entropy of the solvent in different phases. Entropy is the measurement of the disorder dispersal of energy and matter. Liquid phase has higher entropy than the solid phase.

When a pure solvent melts or freezes, the solvent is in both solid-liquid phases, which mean both phases are energetically equivalent. However, the entropy is temperature dependent, the higher the temperature, the higher the entropy. The solute is dissolved in the liquid rather than in solid, this causing the entropy of liquid phase is lowered by the dilution while the entropy of solid phase is unaffected. In the presence of solute, the equilibrium temperature between solid-liquid phases is shifted or more precisely lowered to another temperature. The freezing point depression can be expressed as freezing point depression constant, or cryoscopic constant. The depression of the freezing point of solvent is proportional to the mole fraction of the solute in solution. The freezing point depression can be expressed based on the following equation: ∆T=k[WsoluteMsolute]

Note: ∆T = depression of freezing point (T1 – T2) k = cryoscopic constant

W(solute) = mass of solute per 1000 g of solvent M(solute) = molar mass of solute (g/mol) However, the above equation is only correct for very dilute solution which is ideal. At higher concentration of solute in solution, the above equation will be less accurate due to derivation from ideality.

Apparatus and Material: –

Boiling tubes

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Ice water bath

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Stopwatch

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Thermometer

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20 cm3 and 2 cm3 of pipettes

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Suction bulb

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Retort stand

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Dimethyl Sulfoxide

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Naphthalene

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Unknown liquid

Procedures: Part 1. Determination of freezing point of Dimethyl Sulfoxide 1. 15 cm3 of Dimethyl Sulfoxide (DMSO) was pipetted into a boiling tube.

2. The boiling tube containing the DMSO was immersed in the ice water bath until half of the liquid was already frozen. 3. The boiling tube was taken out from the ice water bath and clamped onto a retort

stand.

4. The temperature of DMSO was measured with thermometer. The temperature of

DMSO was observed and recorded every 0.5 minutes until all the frozen DMSO had fully melted. 5. Then the temperature was further measured for another 3 minutes.

Part 2. Determination of freezing point depression constant 1. 15 cm3 of DMSO was pipetted into a boiling tube.

2. About 0.1 g of naphthalene was weight using analytical balance. 3. The naphthalene was added into the DMSO in the boiling tube. 4. The boiling tube was immersed into the ice water bath until almost half of the solution was frozen. 5. The boiling tube was then taken out from the ice water bath and clamped onto a retort stand. 6. The temperature of the solution was measured and recorded for every 0.5 minutes

until all the frozen solution had fully melted. Then the temperature was further recorded for another 3 minutes. Part 3. Determination of relative molecular mass of unknown liquid 1. 15 cm3 of DMSO was pipette into boiling tube.

2. 2 ml of the unknown liquid was pipette into the boiling tube containing the DMSO. 3. The boiling tube was immersed into the ice water bath until half of the solution was frozen. 4. The boiling tube was then taken out from the ice water bath and clamped onto a retort stand. 5. The temperature of the solution was measured and recorded for every 0.5 minutes until all the frozen solution had fully melted. Then the temperature was further measured for another 3 minutes.

Results: *Table 1: Temperature of Pure DMSO After Taken Out from Ice Water Bath Time (min) Temperature (oC)

4.0 10.0

4.5 11.0

0.0 12.0

5.0 12.0

0.5 11.0

5.5 12.0

1.0 10.0

6.0 12.5

1.5 9.0

6.5 13.0

2.0 9.0

7.0 14.0

2.5 9.0

3.0 9.0

3.5 9.0

7.5 14.0

Table 2: Temperature of DMSO with Dissolved Naphthalene After Taken Out from Ice Water Bath Time (min) Temperature (oC)

4.0 8.0

4.5 8.5

0.0 5.0

5.0 9.5

0.5 4.5

1.0 4.5

1.5 5.0

2.0 5.0

2.5 5.0

3.0 6.0

3.5 7.0

5.5 10.0

Table 3: Temperature of DMSO with Dissolved Unknown Liquid After Taken Out from Ice Water Bath Time (min)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Temperature (oC)

17.0

16.0

15.0

15.0

4.0 13.5

4.5 13.5

5.0 13.5

5.5 13.5

6.0 14.5

6.5 15.0

9.0 19.0

9.5 20.5

10.0 21.0

10.5 21.0

11.0 21.0

11.5 21.0

14.0

7.0 15.0

14.0

7.5 16.0

14.0

8.0 17.0

13.5

8.5 18.5

* Result considered failure; standard freezing point of MDSO will be obtained from another source and used to calculate the freezing point depression constant. Note: –

Graphs were plotted based on the above tables to obtain the freezing point

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The volume of DMSO used was 15 cm3

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Mass of naphthalene used was 0.1061 g

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Volume of unknown liquid used was 2 cm3

Analysis and Calculation: Part 1. Determination of Freezing Point of Pure DMSO From the graph obtained from table 1, the freezing point of pure MDSO is 9.0 oC, but this is significantly fluctuated from the theoretical freezing point of the pure MDSO. Therefore, standard freezing point is used. Freezing point of DMSO = 18.5 oC [2] Part 2. Determination of Cryoscopic Constant Density of DMSO = 1.1 g/cm3 at 20 oC, Volume of DMSO = 15 cm3

Therefore, mass of DMSO = (1.1 x 15) g = 16.5 g Mass of naphthalene = 0.1061 g W (naphthalene) = 6.4303 g per 1000g of DMSO M (naphthalene) = 128.3 g/mol Freezing point of DMSO with dissolved naphthalene = 5.0 oC Freezing point depression = (18.5 – 5.0) oC = 13.5 oC ∆T=k[WnaphthaleneMnaphthalene] 13.5=k[6.4303128.3]

Cryoscopic constant, k = 269.36

Part 3. Determination of Relative Molecular Mass of Unknown Liquid Density of unknown liquid = 0.947 g/cm3 at 20 oC, Volume of unknown liquid = 2 cm3 Mass of unknown liquid used = (0.947 x 2) g = 1.894 g W (unknown) = 114.7879 g per 1000 g of DMSO Freezing point of DMSO with unknown liquid = 13.5 oC Freezing point depression = (18.5 – 13.5) oC = 5.0 oC ∆T=k[WunknownMunknown] 5.0=269.36[114.7879Munknown]

M (unknown) = 6183.85 Relative molecular mass of unknown liquid = 6183.85 ≈ 6184

Discussion: The freezing point of the DMSO obtained from the experiment was fluctuated from the theoretical freezing point of pure DMSO significantly. The theoretical freezing point of DMSO given by Wikipedia was 18.5 oC [2] while the freezing point obtained through experiment was 9.0 oC. The freezing point of pure DMSO obtained was lower than the freezing point of DMSO with added unknown liquid. This has been opposed the colligative property of solution. Due to this fluctuated result, theoretical rather than experimental freezing point had been used to calculate the freezing point depression constant of DMSO. The graphs obtained to determine the freezing point also show unusual shapes. In most of the graphs, the temperatures of the solution in boiling tube were initially decreased, then remain constant followed by increased. U-shaped graphs were obtained rather than theoretical staircase-shaped graphs. Besides, the graphs obtained show that the temperature increased or decreased not in smooth way. There was always more than one constant temperature over the period of time, causing the actual freezing point of the solution hard to be determined. For example, in graph obtained from table 2, the solution remains constant in two part of time. From minute 0.5 to minute 1.0, the temperature of solution remains constant at 4.5 o

C, but the temperature remains constant again from minute 1.5 to minute 2.5 at 5.0 oC.

However, the temperature that remains for the longest period of time was selected as the experimental freezing point of the respective solution. Another unusual phenomenon observed was the shaped of graphs obtained from table 1 and 3. The temperature of solution should be remained constant before the solid phase was disappeared due to the solid-liquid equilibrium when the freezing point was achieved. However in the two graphs, the temperature of solution increased rather than remains constant even though the solid phase was still present in boiling tube

One of the possible errors that occurred was the improper method in measuring the temperature of the liquid in boiling tube. The liquid in the boiling tube might be unevenly stirred when the temperature of the liquid was taken and recorded. Besides, another possible error that occurred was the accidentally addition of other substances into pure DMSO before and while the temperature was recorded. This might happened due to the contamination of either the pipette or the boiling tube or both. These two apparatus might not be properly cleaned by us before and during the experiment. Contamination in pure DMSO can leaded to big fluctuation in freezing point. The same errors might have occurred for the measurement of depressed freezing point of DMSO with dissolved naphthalene and unknown liquid. The only precaution step that may be taken to prevent the above errors is to rinse the apparatus with water and distilled water more frequently before using them to minimize the contamination occurrence. Besides, the solution in boiling tube should be stirred all the time while the temperature was recorded to prevent any uneven distribution of heat in the solution.

Conclusion: The relative molecular mass of the unknown liquid was about 6184. The value calculated was just an approximation with significant deviation from the actual relative molecular mass of unknown liquid.

References: 1. Siska P, E. (2006). University Chemistry. Pearson Benjamin Cumming.

2. Dimethyl Sulfoxide. (n.d.). Retrieved March 1, 2009, from http://en.wikipedia.org/wiki/Dimethyl_sulfoxide

3. Freezing Point Depression. (n.d.). Retrieved March 1, 2009, from http://en.wikipedia.org/wiki/Freezing-point_depression

Encl.: Graph in determination of freezing point of pure DMSO Graph in determination of freezing point of DMSO with added naphthalene Graph in determination of freezing point of DMSO with added unknown liquid Raw data