Lab Report 3 - Solution and Freezing Point PDF

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Solution and Freezing Point...

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Solutions and Freezing Point Introduction: Molality is the number of moles of solute per kilogram mass of solvents. It is expressed in mol/kg. Molality of the solutions could also be determined from change in temperature per molal freezing point depression constant of the solvents. The purpose of the experiment was to find the molar mass of unknown substance through molality of the solution. It was hypothesized that the molality of the solutions could be used to determine molar mass of unknown.

Experiment: To determine the molality of unknown, test tube, thermometer, 500 ml beaker, 50 ml beaker and hot tab were used. The experiment was performed in hood. At first, the mass of empty 500 ml beaker was weighted and mass of test tube along with beaker was measured. About 400 ml of water was poured in 500 ml beaker and placed above the hot tub. The temperature was increased. About 8.09 gram of benzophenone was measured in 50ml beaker and kept inside test tube. The test tube with benzophenone was kept inside 500 ml beaker. Test tube was immersed partially inside water and thermometer was inserted inside test tube to measure temperature. The test tube was supported with stand and thermometer with clamp. The increased temperature caused benzophenone to melt and corresponding temperature was noted. The melted mixture was allowed to cool by taking test tube out of the beaker. the mixture was stirred with stirrer until it completely freezed and temperature was noted every 10 seconds. After the mixture was completely cooled, unknown solution was added. The mixture with unknown was kept inside test tube in hood and allowed to melt. when the mixture was melted, the corresponding temperature

was noted and same process was repeated. Finally the mixture changed into solid and it was then allowed to melt and washed with acetone.

Result: The molar mass of unknown could be determined by using molality formula. At first, the mass of empty test tube was determined by subtracting the mass of beaker from mass of beaker and test tube. Mass of empty beaker=211.65g Mass of empty beaker+ mass of test tube = 263.90g Mass of empty test tube= (mass of beaker + mass of test tube) – mass of empty beaker = 263.90g- 211.65g =52.3g Benzophenone was used as solvent and unknown solution was used as solute. The mass of unknown was about 1.41 grams. Mass of benzophenone= 8.09g = 0.00809 kg Mass of unknown= 1.41 g The freezing point of benzophenone was noted when temperature remained constant every 10 seconds during freezing and the freezing point of benzophenone and unknown was noted when the temperature of benzophenone and unknown remains constant every 10 seconds during freezing. Hence, the freezing point of benzophenone was found 48.3℃ and freezing point of benzophenone and unknown was found 34.7℃.

Freezing temperature of benzophenone=48.3℃ Freezing temperature of benzophenone and unknown= 34.7℃

The difference of freezing point of benzophenone and freezing point of benzophenone and unknown gives the change in freezing point. The molality of solution could be found by dividing change in temperature by molal freezing point depression constant of the solvent. The change in freezing point (ΔTf ) = Tbenzophenone - Tbenzophenone+ unknown = 48.3℃ - 34.7℃ =13.6℃

Molal freezing point depression constant of the solution (Kf) =9.8

Molality of solution =

ΔT f Kf

=13.6/9.8 1.39 The moles of unknown was calculated using molality of solution and mass of solvents in kilograms. The molality of solution was calculated from temperature and molal freezing pount depression constant of the solvent and mass of solvent i.e., benzophenone was known. So, the moles of unknown was given by, Moles of unknown = molality *mass of benzophenone =1.39*0.00809 0.0112451mol

From the value of moles of unknown, the molar mass of unknown could be determined. The molar mass is mass of substance per mole. Therefore, the molar mass of unknown was mass of unknown per mole of unknown. Molar mass of unknown =mass of unknown/ moles of unknown =1.41 g/ 0.0112451 =125.38g/mol The table and graph represent change in freezing temperature of benzophenone and freezing temperature of benzophenone and unknown every 10 seconds. After certain period, the change in temperature was found constant which was the freezing point of benzophenone and unknown. Time (seconds )

10 20 30 40 50 60

Freezing temperature of Benzophenon e (℃) 59 57.5 56 54.4 53.3 52.3

Freezing temperature of Benzophenon e + unknown (℃) 62.1 61.1 59.7 58.2 57 55.8

Time vs Temperature 64 62 60 58 56 54 52 50 48 46 0

10

20

30

40

50

60

70

Freezing temperature in Benzophenone Freezing temperature of Benzophenone + unknown

Conclusion: The molar mass of unknown was determined from molality of the solution. The molar mass was found to be 125.38g/mol which was quite like molar mass of iron. It is expected that the unknown substance was iron. In this experiment, the solvent was heated and then cooled till freezing point and unknown substance was mixed with solvent. The same process was repeated for unknown substance. The molality of solutions helps to determine the mole of unknown. Finally, the molar mass of unknown was determined. The experiment could be used in future to determine the molar mass of other unknown compounds. There may be inaccuracy in the results due to inappropriate measurements. The problem could be reduced to find the precise value of unknown....