Postlab #1

Daniel D. LeeTA: Courtney GuentherCHEM 245-102February 3rd, 2011

Lab Report #1SOLIDS: Recrystallization and Melting Points

Section 4: Results and ObservationsMelting Point DeterminationMel-Temp apparatus #6, Correction:

Naphthalene (Sample A):Melting Ranges (°C)

Trial #1 78.0 – 80.0- (-1.0) (-1.0) Mel-Temp apparatus #6 corrections 79.0 - 81.0°C Temperature Observed, corrected

Trial #2 77.0 – 80.0- (-1.0) (-1.0) Mel-Temp apparatus #6 corrections 78.0 - 81.0°C Temperature Observed, correctedBoth trials within 1°C, ✓ (True high melting point temperature: 81°C)

Benzil (Sample C):Melting Ranges (°C)


Trial #2 90.0 – 95.0- (-1.5) (-1.5) Mel-Temp apparatus #6 corrections 91.5 - 96.5°C Temperature Observed, correctedBoth trials within 1°C, ✓ (True high melting point temperature: 94°C)

Cinnamic Acid (Sample D):Melting Ranges (°C)


Trial #2 128.0 – 130.0- (-1.5) (-1.5) Mel-Temp apparatus #6 corrections 129.5 - 131.5°C Temperature Observed, correctedBoth trials within 1°C, ✓(True high melting point temperature: 132°C)

Unknown (Sample 119):Melting Ranges (°C)

Trial #1 (Preliminary)

147.0 – 149.0- (-1.5) (-1.5) Mel-Temp apparatus #6 corrections 148.5 - 150.5°C Temperature Observed, corrected

Trial #2 149.0 – 151.0- (-1.5) (-1.5) Mel-Temp apparatus #6 corrections 150.5 - 152.5°C Temperature Observed, correctedOut of the 12 possible samples and according to their given upper end of melting ranges, the observed melting point range of the unknown can either be adipid acid or benzenesulfonamide since their upper end of melting point range is around 150°C.

Mixture-melting point determinations with adipid acid (Sample N) and benzenesulfonamide (Sample O):

Melting Ranges (°C)Sample N 128.0 – 135.5

- (-1.5) (-1.5) Mel-Temp apparatus #6 corrections 129.5 - 137.0°C Temperature Observed, corrected

Sample O 149.0 – 151.5- (-1.5) (-1.5) Mel-Temp apparatus #6 corrections 150.5 - 153.0°C Temperature Observed, correctedThe melting point range of the mixed sample of vial O and the unknown reflects the initial melting point measurement of the unknown.

RecrystallizationBenzoic acidInitial Weight 0.90 gProduct Weight 0.23 gPercent Recovery Percent Recovery=(Weight of pure crystals recoveredWeight of original sample )×100

Benzoic Acid:(0.23g/0.90g)x100 = 25.56 %

Melting Point Range


- (when heating the impure benzoic acid with the necessary solution, the sample boiled out of the flask because the heat was set too high) – potentially sample was lost

- After heating, the solution turns gray/dark.- As the sample cools down, it solidifies pretty quickly into white powdery

figures

AcetanilideInitial Weight 2.06 gProduct Weight 0.84 gPercent Recovery Percent Recovery=(Weight of pure crystals recoveredWeight of original sample )×100

Benzoic Acid:(0.84g/2.06g)x100 = 40.78 %

Melting Point Range


- When added to boiling water, the resulting sample is a very clear blue solution.

- Weight of decolorizing carbon: 0.23go Turned the solution black, and after hot filtration the solution became

clear.- The product was clear/white powdery crystals.

Thermometer CalibrationTemperature Observed from Iced Water 1.0°C

Temperature Observed from Boiling Water 98.0°C

Barometer Reading 757.8 mm Hg

Corrections At 0°C, -1°CAt 100°C, +2°C

Section 5: Conclusions Melting Point Determination

In obtaining the melting points of naphthalene, benzil, and cinnamic acid, I was able to practice using the Mel-Temp apparatus. For the observed melting ranges, they were close to that of the true temperature ranges: the naphthalene’s two trials’ observed upper melting point matched that of the true value’s, the benzil’s two trials’ both showed high melting temperature 2.5°C greater than the true temperature, and cinnamic acids’ both trials showed values that were 0.5°C less than that of the true values. For each case, the measured values were relatively close to the true values, and both trials for each compound were less than 1°C off each other. Because the measured values were greater than the actual melting point range for the benzil, less than for the cinnamic acid, and same for naphthalene, we cannot conclude that the correction for the Melt-Temp apparatus is incorrect; rather, it is possible that determining when melting actually ended for each compound, the vague visual cues, may have been the cause for the small deviations.

Measuring the melting range for Unknown 119, the melting ranges came out to be 148.5-150.5°C and 150.5-152.5°C, reflecting the upper end of melting ranges of

adipic acid and benzenesulfonamide, 150°C and 151 respectively, and indicating that the unknown sample be one of them. Then, with the mixture-melting point process, for which the two possible samples were each mixed with the unknown in approximately equal proportions to get the melting ranges of the mixed samples, the melting range of adipid acid and the unknown was 129.5-137.0°C, less than when the unknown was measured previously. On the other hand, the mix of the benzenesulfonamide and the unknown had a melting range (150.5-153.0°C) that was consistent with the unknown’s individual measurement. Therefore, it can be concluded that the unknown sample 119 was benzenesulfonamide since is it generally the case that impurity decreases the melting point of a pure solid.

RecrystallizationFor recrystallizing the impure benzoic acid, the product weight was 0.23 g

while the initial weight was 0.90 g. The percent recovery was 25.56% ad the melting range of the product was 113.5-117.5°C. As for the percent recovery, if it is less than around 20%, the experiment is considered unsuccessful, having lost too much of the desired sample during the process. Even though 25.56% is not within this unfavorable range, it is still pretty close to it. This may have been due to products lost during filtrations or may have been due to it spilling over when it was boiling at the beginning of the experiment. Also, considering how the true upper melting range of benzoic acid is 122°C, the observed 113.5-117.5°C range is pretty low. This may have also been due to the filtration process that was not able to completely purify the sample. The hot gravity filtration process may have let some impure solutes to pass the filter paper, possibly due to improper folding of the fluted filter paper or maybe the crystallized sample was not cleanly washed enough with cold solvent right after the vacuum filtration to wash away the soluble impurities that may have been with the wetness of the product.

As for the acetanilide, the initial weight of the sample was 2.06g and the product weight was 0.84g. The percent recovery was 40.78% and the melting range of the product was 108.5-114.5°C. The percent recovery of the experiment indicates that a fair amount of the original sample was able to recrystallize and to purify to become products. Also, compared to the true upper melting range of acetanilide of 114.3°C with the observed 108.5-114.5°C, the two upper melting range is very close. However, the range of the observed is a bit too large, which is far greater than the ideal 2°C range, indicating some sort of impurity or error in measurement of the melting range via the Mel-Temp apparatus.

As for the thermometer calibration, if the temperature from this specific thermometer is observed to be 1°C, then subtract 1°C to correct it because the actual temperature is probably 0°C. If the temperature is observed to be 98°C, add 2°C to correct it because the actual temperature is probably 100°C. Technically speaking, to get the actual temperature, at 0°C, subtract 1; at 100°C, subtract -2.

Section 6: Answers to Assigned Questions

Exercise 20) For the benzoic acid, 0.9 grams of benzoic acid was recrystallized from 20 mL of water. With solubility of 0.02 g per 100 mL at 0°C, we expect 0.004 g to be

the amount of unrecoverable by virtue of its solubility at 0°C. For the acetanilide, 2.06 grams of acetanilide was recrystallized from 40 mL of water. With solubility of 0.53 g per 100 mL at 0°C, we expect 0.212 g to be the amount of unrecoverable by virtue of its solubility at 0°C. Calculations for benzoic acid: 0.02g/100mL = x g/20mLCalculations for acetanilide: 0.53g/100mL = x g/40mL

Exercise 23)a) In the solution step, an unnecessarily large volume of solvent is used.

a. Using excessive amounts of solvent decreases the recovery of the solute because it will waste time in boiling off later in the process and if too much solvent is used for cooling later on after filtration, little or no solutes crystallizes out. More solvent means more of the solutes will stay dissolved.

b) The crystals obtained after filtration are not washed with fresh cold solvent before drying.

a. The extra fresh cold solvent will allow for the sample to become purer by washing away the soluble impurities that may be present after filtration because the crystals are not completely dry yet. After boiling off, the crystals may still have solvent or water pockets that were able to stay with the crystals, which would have soluble impurities that need to be washed with fresh cold solvent before drying.

c) The crystals referred to in (b) are washed with fresh hot solvent.a. If the crystals are washed with fresh hot solvent, the crystals are in

danger of being dissolved again, eventually leading to a lost of the desired product.

d) A large quantity of decolorizing carbon is used.a. Even though it is able to suck up impurities, it will also absorb some of

the desired products if too much of the decolorizing carbon is used.e) Crystals are obtained by breaking up the solidified mass of an oil that

originally separated from the hot solution.a. That oil will probably contain some impurities. When the separated

oil cool down, the oil crystallizes; however, because the oil was part of the solution before separating, it is likely that the oil dissolved some of the solvent and impurities there within. Therefore, obtaining crystals from breaking up the solidified mass of an oil that originally separated from the hot solution is not preferable.

f) Crystallization is accelerated by immediately placing the flask of hot solution in an ice-water bath.

a. If the hot flask is immediately placed in an ice-water bath, the flask may shatter. Also, if the solution is not cooled slowly, it may result in rapid recrystallization of small crystals that may trap impurities.

Exercise 14)a) Melting range of about 2°C is considered normal and pure.

b) Because the sample decomposed while melting, it is inferred that the sample is impure. Therefore, the melting point is lowered as a result. In this case, the 147°C (dec) not only is not in the format of melting ranges, it is also decomposition while melting, indicating some sort of impurity.

c) 46-60°C melting range would be considered very impure as the range of 14°C is far greater than the ideal 2°C range.

d) 162.5-163.5°C melting range indicates a pure sample as the range is only within 1°C, unless it is a mixed sample at its eutectic point which is a rare phenomenon.

Exercise 16)Mixture-melting point procedure will use the pure samples to identify the unknown. By mixing the pure samples of the three possibilities with the unknown in equal proportions, the melting ranges of the mixed samples will be measured using the Mel-Temp apparatus. The sample that again results in the melting range around 160°C will indicate that the unknown is the same as the sample mixed with it in the mixture. The other two samples will probably have melting ranges that are less than the previously observed 160°C.

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Postlab #1