Recitation for Exp 3 - Work PDF

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Recitation Problems for Exp. 3 Identifying a Constituent of “Panacetin” Name _________________________________________________

Section _______________________

Recitation problems may be collected randomly for a grade. 1. Successful purification by recrystallization is based primarily on the solubility of a solute in a solvent. With the water solubility values given, which one or more of these solutes would recrystallize effectively in water? If not, identify the specific experimental issue you would encounter when recrystallizing.

0.3, 0.2,0.1 g/100ml are good solubilities in water; 0.0009g/ml is too low and it wouldn’t increase a lot with the temperature (won’t dissolve), 24.99g/100ml is too high and it wouldn’t change a lot with temperature too—loss of solid by solubility (too much will dissolve). 2. Suggest 3 reasons why we employ water instead of ethanol as the recrystallization solvent for the unknowns? Not expensive?; difference between boiling point and room temperature is larger, loss via evaporation is less? 3. Assuming you have 1.4 g of unknown recovered from Exp. 2, what are the minimum amounts of cold and hot water needed to dissolve that mass of both acetanilide and phenacetin? Why is phenacetin’s water solubility different than that of acetanilide. Acetanilide -- solubility(cold water) = 0.54g/100mL, solubility(boiling water) = 5.0g/100mL; phenacetin -- solubility(cold water) = 0.076g/100mL, solubility(boiling water) = 1.22g/100mL 1.4g(100ml/5g) acetanlilide= 28mL boiling water 1.4g(100ml/0.54g) acetanilide= 259.3mL cold water 1.4g(100ml/1.22g) phenacetin= 114.8mL boiling water 1.4g(100mL/0.076g) = 1842.1mL cold water

4. How do you decide whether to use enough hot water to dissolve acetanilide or enough to dissolve phenacetin? Determine which temperature is acetanilide and phenatin more soluble in, in this case it is boiling water. 5. If someone mixed sand into your unknown sample so you could not separate the grains of sand from the unknown solid, how could you separate them? Would the same action work if sucrose was mixed into your unknown? Dissolve the unknown again and filter the sand out. We did the same with sucrose, since sucrose isn’t soluble with dimethyl chlorine. 6. Why would you allow the filtered liquid to cool slowly? What would happen if you did not allow slow cooling? Cooling slowly allows for recrystallization of purer crystals to occur, cooling them faster would mean that the recystralization process is impure. 7. Why and when would you put the recrystallizing solution in an ice bath? Put it in after the unknown mass is dissolved, put it in an ice bath to allow recrystallization. 1. Why would we use capillary tubes filled with such a small amount of solid to determine the unknown’s melting point as opposed to observing larger amounts of solid melt on a hot plate? Large amounts would take longer and amounts don’t influence melting point. 2. Why is the melting point reported as a range? How is the range decided? Melting point is never exact because of impurities—they cause it to have a range. The range is considered from the first drop of liquid to all liquid. 3. Why should you obtain at least two melting point readings per sample? If your first two melting point values don’t agree, what explanations might you suggest?

Two melting point readings per sample allow for accurate readings; if they don’t agree, then there are impurities. 4. Why are you asked to measure the melting point of the unknown, the unknown mixed 1:1 with pure acetanilide, and the unknown mixed 1:1 with pure phenacetin? Mixing it with the three samples means that the actual value of the unknown is obtained. Comparing it with the acetanilide and phenacetin means that change may occur. If it doesn’t, then it is the same as the one that it is mixed in. 5. Would a small amount of phenacetin (m.p. 135 °C) in a sample of predominantly acetanilide (114 °C) increase or decrease the melting point and why? It would decrease acetanilide, even though it has a higher melting point. Impurities always lower and broaden the range of a melting point. More Sophisticated Instruments Available to a Chemist: -NMR – can identify the number and position of atoms on an organic compound (especially hydrogens and carbons), important for identifying chemical structure. Each instrument run takes approximately 5 minutes and requires 5-10 mg of analyte. -GCMS – separates components of a volatile mixture by their boiling point and polarity, then identifies them by their mass. Each instrument run can take from 5-25 minutes and requires much less than 1 mg, more like a nanogram of analyte. Can search results against libraries of spectra to quickly find the identity of the analyte. -LCMS – same as GCMS, but separates components in liquid phase instead of gas phase -IR – can identify covalent bonds on a molecule by their characteristic absorbance of infrared energy. Each instrument run can take 1-3 minutes and requires 1-10 mg of analyte. The instrument is portable and allows identification by searching against a library of spectra to quickly find the identity of the analyte.

Real world situation: (choose the best instrument for the situation and list a reason why) 1. An ink sample, collected at a crime scene, contains a unique set of organic dyes and chemicals that could identify the brand and type of pen used. GCMS: low molecular mass allows for GCMS to be used 2. Paint chips found at the scene of a hit run need to be identified immediately to aid in apprehending the vehicle. IR spectra: used as a fingerprint to compare cars of similar color paint to the paint chip color that is identified with IR spectra. 3. A patient’s blood sample may contain tumor markers that signal a high probability of cancer. LCMS: large molecules at low concentration, the molecules are nonvolatile 4. A drug lead was synthesized in a pharmaceutical lab and the researcher is trying to discover if his reaction led to the correct product. NMR: specific molecular structure 5. Cattle farmers suspect that a nearby water source is contaminated with pesticide run-off from an adjacent farm and is causing health issues in their cattle. GCMS: pesticides are small molecules and in small concentration 6. A chemical waste/safety officer forgot to label two liquid bottles and needs to quickly determine their identity. IR: functional groups present in compound to label unknown liquids...