Experiment: 22Title: Determination of Relative Molecular Mass by Endpoint CryoscopyObjective:Determine the freezing point of Dimethyl Sulfoxide (DMSO).Determine the freezing point depression constant when naphthalene dissolved inDMSO.Determine the relative molecular mass of an unknown liquid from the freezing depression constant.Introduction:Freezing point depression is one of the colligative properties of solution, meaning thatthis property is universal to all solutes in all solution regardless the identities of thesolventandsolute.It alsoregardless whether thesoluteis electrolyteor nonelectrolyte.The freezing point depression is only dependent on the presence of solute in solvent andtheir quantities.When there is solute dissolved in a solvent forming a solution, the freezing point of thesolution is lower than the pure solvent due to the presence of added solute. Freezing pointdepression in a solution is due to the entropy of the solvent in different phases. Entropy isthe measurement of the disorder dispersal of energy and matter. Liquid phase has higherentropy than the solid phase.When a pure solvent melts or freezes, the solvent is in both solid-liquid phases, whichmean bothphases areenergetically equivalent. However,the entropy istemperaturedependent, the higher the temperature, the higher the entropy. The solute is dissolved inthe liquid rather than in solid, this causing the entropy of liquid phase is lowered by thedilution while the entropy of solid phase is unaffected. In the presence of solute, theequilibrium temperature between solid-liquid phases is shifted or more precisely loweredto another temperature.The freezing point depression can be expressed as freezing point depression constant, orcryoscopic constant. The depression of the freezing point of solvent is proportional to themole fraction of the solute in solution. The freezing point depression can be expressedbased on the following equation:T=k[WsoluteMsolute]Note:T= depression of freezing point(T1 T2)k = cryoscopic constantW(solute) = mass of solute per 1000 g of solventM(solute) = molar mass of solute (g/mol)However, the above equation is only correct for very dilute solution which is ideal. Athigher concentration of solute in solution, the above equation will be less accurate due toderivation fromideality.Apparatus and Materials :

Boiling tube, salt / ice / water bath, stopwatch, thermometer (-20 C to + 30 C, graduated in 1 / 10 C), stirrer, 20 cm3 and 2 cm3 pipettes (both fitted with suction bulb), benzene, naphthalene, unknown liquid.

Experimental Procedure

Benzene is used as the solvent in this experiment because it is readily available, possesses a convenient freezing point, and has a relatively large cryoscopic constant.

a) Determination of freezing point of pure solvent. Pipette exactly 20 cm3 of benzene into a pyrex boiling tube and cool it in a salt / ice / water bath until about one half of it has frozen. Then clamp it vertically, insert a (1 / 10 C) thermometer, stir gently and note the temperature at 1 / 2 minute intervals until all the solid has melted and then for a further 3 minutes. The melting portion of the (temperature versus time) curve should be flat for a pure substance.

b) Determination of cryoscopic constant of benzene. The freezing point of pure benzene (T0) is obtained in (a). Add an accurately known mass of naphthalene (ca. 1.0 g) to the 20 cm3 of benzene used in (a) and determine the melting curve for the solution as described in (a).

c) Determination of relative molecular mass of unknown compound. Pipette exactly 2 cm3 of the unknown liquid into a boiling tube containing exactly 20 cm3 of benzene. Determine the freezing point of this solution as described in (a) ( = Tu).

Results:*Table 1: Temperature of Pure DMSO After Taken Out from Ice Water BathTime(minute)0.00.51.01.52.02.53.03.54.0

Temperature(c)12.011.010.09.09.09.09.09.010.0

4.55.05.56.06.57.07.5

11.012.012.012.513.014.014.0

Table 2:Temperature ofDMSO withDissolved Naphthalene AfterTaken Outfrom Ice Water BathTime(minute)0.00.51.01.52.02.53.03.54.0

Temperature(c)5.04.54.55.05.05.06.07.08.0

4.55.05.5

8.59.510.0

Table 3: Temperature of DMSO with Dissolved Unknown Liquid After Taken Out from Ice Water BathTime(minute)0.00.51.01.52.02.53.03.54.0

Temperature(c)17.016.015.015.014.014.014.013.513.5

4.55.05.56.06.57.07.58.08.59.09.5

13.513.513.514.515.015.016.017.018.519.020.5

10.010.511.011.5

21.021.021.021.0

Result considered failure; standard freezing point of MDSO will be obtained fromanother source and used to calculate the freezing point depression constant.Note:Graphswereplotted based onthe above tablesto obtainthe freezing pointThe volume of DMSO used was 15 cm3Massof naphthaleneused was0.1061 gVolume of unknownliquid used was2 cm3

Calculation :Part 1. Determination of Freezing Point of Pure DMSO from the graph obtained from table 1, the freezing point of pure MDSO is 9.0C, but thissignificantly fluctuatedfromthetheoreticalfreezingpointofthepureMDSO.Therefore, standard freezing point is used. Freezing point of DMSO = 18.5C [2]Part 2. Determination of Cryoscopic Constant Density of DMSO = 1.1 g/cm3 at 20C, Volume of DMSO = 15 cm3Therefore, 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 DMSOM (naphthalene) = 128.3 g/mol Freezing point of DMSO with dissolved naphthalene = 5.0c Freezing point depression = (18.5 5.0)oC = 13.5CT=k[WnaphthaleneMnaphthalene]13.5=k[6.4303128.3]Cryoscopic constant, k = 269.36 Part 3. Determination of Relative Molecular Mass of UnknownLiquid Density of unknown liquid = 0.947 g/cm3 at 20C, Volume of unknownliquid=2cm3Mass of unknown liquid used = (0.947 x 2) g= 1.894 gW (unknown) = 114.7879 g per 1000 gof DMSO Freezing point of DMSO with unknown liquid= 13.5oCFreezing point depression = (18.5 13.5)oC= 5.0CT=k[Wunknown Munknown]5.0=269.36[114.7879Munknown]M (unknown) = 6183.85Relative 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 ofDMSO given by Wikipedia was 18.5C [2] while the freezing point obtained through experiment was 9.0C. The freezing point of pure DMSO obtained was lower than thefreezingpointofDMSOwithaddedunknownliquid.Thishasbeenopposedthe colligative property of solution. Due to this fluctuated result, theoretical rather than experimental freezing point had been used to calculate the freezingpoint depression constant of DMSO. The graphs obtained to determine the freezing point also show unusual shapes. In most ofthe graphs, the temperatures of the solution in boiling tube were initially decreased, thenremainconstantfollowedbyincreased.U-shapedgraphswereobtainedratherthantheoretical 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 oftime,causing theactual freezingpointofthe solution hardtobedetermined.Forexample, in graph obtained from table 2, the solution remains constant in two part oftime. From minute 0.5 to minute 1.0, the temperature of solution remains constant at 4.5C, butthe temperature remains constant again fromminute 1.5to minute 2.5at 5.0C. However, the temperature that remains for the longest period of time was selected as the experimental freezing point of the respectivesolution. Another unusual phenomenon observed was theshaped of graphs obtained from table 1and 3. The temperature of solution should be remained constant before the solid phasewasdisappearedduetothesolid-liquidequilibriumwhenthefreezingpointwasachieved.Howeverin thetwo graphs, thetemperature ofsolution increased rather than remains constant even though thesolid 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,anotherpossibleerrorthatoccurredwastheaccidentallyadditionofothersubstances 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 leadedto big fluctuation in freezing point. The same errors might have occurred for the measurement of depressed freezing point ofDMSO 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 contaminationoccurrence. Besides, the solution inboiling tubeshould bestirred 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:-PETER HONG LEONG CHEAH. Exp 4. [online]. Exp 4. Available at: -https://www.scribd.com/doc/18026453/Exp-4. Use on 1st april 2015.

UDEC 1134 CHEMISTRY LABORATORY 1

NAME

BERNARD CHEONG JIA CHOON

NAME OF GROUP MEMBERSTANG YI ZHANG, PAN WAI SENG

STUDENT ID 1406645

NO. OF EXPERIMENTEXP 22

TITLE OF EXPERIMENTCryoscopy: Determination of relative molecular by endpoint cryoscopy

DATE OF EXPERIMENT6/04/2014

PRACTICAL GROUPP2

LECTURERMS CHANG CHEW CHEEN