CHM 205 2023 with 100% correct questions and answers

Why would you use a filtering pipet? Used to remove unwanted solid from a liquid -When you want the liquid Why would you use a filter-tip pipet? -to remove solids from very small amounts of liquid by leaving solid behind (as opposed to trapping it) -when you want the solid Why (in exp. 0) do you rinse w/ 1 mL of methanol and 1 mL of hexane? -methanol dissolves and hhexane has a low bp to evaporate liquid What m.p. indicates purity? -a narrow melting point range -an impurity lowers and broadens m.p. -to lower freezing point, add impurity as well What does mixed melting point do? -If the unknown and known are the same, then there is no change in melting point range - if not, then the range will react like an impurity was added Melting point range temperature of first liquid- temperature of completely melted When using the mel-temp, why shouldn't you heat faster than 1-2 ˚C? -can result in misread temperature due to crystal lagging What does it mean if the solution in the mel-temp turned from white to brown? -Decomposition occurred What happened if the solution in the mel-temp disappeared? -sublimation occurred, seal capillary What is the literature m.p. value for ferrocene? -173-174˚C What is the literature m.p. value for adipic acid? -151-154˚C What is the literature m.p. value for acetyl ferrocene? -81-86˚C What is the literature m.p. value for citric acid? -153-159˚C How do you calculate the number of drops from a given mL? -x mL x # drops/ mL = x drops How do you calculate the number of drops from a given weight (g)? - X g x cm^3/g x # drops/mL = x drops How do you calculate the # g from a given number of drops? - # drops x 1 mL/ 28 drops = # mL x g/mL= # g Why should you always add boiling chips before heating them to their boiling point? -to prevent bumping and local overheating (decreased evaporation) Why should you try to dissolve a compound in as little solvent as possible? (during recrystallization) -to increase compound crystallization from increased concentration How are insoluble impurities removed? -from hot gravity filtration How are soluble colored impurities removed? -from charcoal and subsequent HGF Why should you never add charcoal (or an solid substance) to a boiling solution? -it could boil over why do you scratch the flask? (in macroscale recrystallization) -to create a surface for nucleation After macroscale recrystallization, what are two ways you can get more recovery? -cool further for more recovery -evaporate for a higher concentration What is the purpose of CaSO4? -to dry solution/ absorb moisture What is the solvent/solute used in the macroscale recrystallization? -water and organic unknown When would you use microscale recrystallization? -for ranges 10-100mg in .5-.15 mL of solvent (in this case it's water) Why do you rotate the microspatula rod while adding water? -to improve dissolution Why, in microscale recrystallization do you want to avoid the presence of residual charcoal? -Will interfere with % yield and proper ID of substance If crystals haven't appeared in microscale recrystallization, what should you do? -reduce the volume of filtrate by ~20%, or until the crystals begin to appear in the heated solution What do you use instead of vacuum filtration in microscale recrystallization? -centrifugation The solubility of an unknown compound X in ethanol is found to be 0.21 grams per 10 mL at 0 °C and 1.4 grams per 10 mL at 78°C. What is the minimum amount of ethanol needed to recrystallize a 2.0 gram sample of compound X? After recrystallization of the 2.0 gram sample, how much of compound X will remain dissolved in the cold solvent (i.e., how much of compound X is not recovered after the recrystallization)? 2.0 grams X * 10 mL ethanol / 1.4 grams X = 14.29 mL of ethanol 0.21 grams X / 10 mL ethanol * 14.29 mL ethanol = 0.30 grams X remain dissolved in the cold solvent Why is it important to use only the minimal amount of hot solvent needed to achieve a recrystallization? What would be the effect of using too much solvent while dissolving a solid for recrystallization? It is important to use minimal amount of hot solvent because if the amount of hot solvent exceeds 25 mL, then it will be more difficult to recover the crystals. Upon cooling there will be very little actual recrystallization. What do you think would happen to your recrystallization solution if you did not use a preheated filter during the "hot" gravity filtration step If a pre-heated filter wasn't used during the "hot" gravity filtration step, then when the hot solution touched the cold (or room temperature) filter paper, there is a risk that the crystals would crystallize on the fluted filter paper. Why is benzyl alcohol a poor choice of solvent for recrystallization of fluorenol? -its b.p. is too high -a solute must be dissolved near b.p. of solvent to increase saturation when cooled, if mp of solute lower than bp of solvent, then solute will turn to oil and will be harder to dissolve and achieve yield why is acetone a bad solvent? -low bp and flammable how do you remove a very soluble contaminant? -remains with mother liquor following vacuum filt and centrifugation What is the partition coefficient? -accords distribution of mixture between two immiscible solvents -KD = C2/ C1 What solvent do we use in extraction? -methyl tert-butyl ether (MtBE) bc less flammable than diethyl ether (effective solvent) Why would you use the liquid-liquid extraction method? separate compounds making up a mixture using to immiscible solvents what happens if you add a base (NaOH) to water containing RCO2H? -solubility increases under basic conditions? -Turns to RCO2Na, which is a salt and water soluble How do acids & phenols increase in solubility? -by addition of a base Why would separation not be possible if aqueous NaOH was used instead of NaHCO3 in a mixture of carboxylic acid and phenol? Because NAOH would deprotonate both and both would reside in the aqueous base layer How are amines extracted? with the addition of acid Why do we add anhydrous sodium sulfate? As a drying agent to remove traces of water from the organic layer we dry for `5 minutes What do you use to remove the solvent? -a warm sand bath and a gentle stream of air What is the solute in the extraction experiment? -Benzoic acid What layer is the organic layer? -Top layer, has benzoic and MtBe If you were trying to separate a sample mixture with base, carboxylic acid, and neutral compound, what would you use? -Acid If you were trying to separate a sample mixture with carboxylic acid, phenol, and neutral compound, what would you use? -requires weak base What is back extraction? -adding MtBe to a separated solvent to back extract any compound that should not have gone into the aqueous layer in a neutral compound, what kind of solvent would we use? -h2o to dissolve any kind of aqueous dissolving stuff Extraction scheme for separating an organic base with an organic neutral compound Dissolve the base and neutral compound in an organic solvent Add HCl unitl the solution becomes acidic Separate the organic layer and aqueous layer carefully Dry the organic layer to obtain neutral compound Add NaOH and an aqueous solvent to the aqueous layer Remove the organic layer and evaporate to obtain the base Extraction scheme for separating an organic acid and phenol Dissolve acid and phenol in organic solvent Add NaOH Separate the organic and aqueous layer carefully Add HCl to the aqueous layer Add organic solvent to the aqueous layer Evaporate the organic layer to obtain acid Add HCl to the aqueous layer and remove the aqueous Add organic solvent and NaOH to the aqeous layer Remove the organic layer and evaporate to obtain phenol If you aren't sure which layer is which what can you do To see what layer is what add a small drop of water and put it in the test tube and see where it goes why should potassium carbonate not be used with some classes of organic compounds? RNH2 would be a better choice to use with potassium carbonate because if you use rcooh it forms water What is used to make the TLC plate? Silica gel, SiO2 -stationary phase What is the mobile phase in thin layer chromatogahpy? the solution What determines the motility of solution? 1. polarity of the solvent (mobile phase 2. polarity of the compound (solute) 3. polarity of adsorbant * more polar the solute = greater affinity it will have for stationary and less it will travel* What are the two most common adsorbants in tlc? silica gel and alumina What is Rf? -retardation factor -distance traveled by the compound to that of the solvent front -C2/ C1 What is the solute used in Part A of tlc analysis? -orthohydryoxyacetophenone and para-hydroxyacetophenon Why do o- and p hydroxy have a big dif. in mP? -O hydroxy has hydrogen bonding and a delocalized e- pair = lower MP What does an rf of .91 indicate? - use less polar solvent what does an rf of .15 indicate? -use more polar solvent List in order of decreasing polarity: naphthalene, fluorenol, o-toluic acid o-toluic acid, fluorenol, naphthalene = Rf: naphtalene, fluor, o-toluic arrange the solvents in order of increasing polarity: ethanol - ethyl acetate - n-propanol - dichloromethane - toluene - heptane heptane-toluene-dichloromethane- ethyl acetate- n-propanol- ethanol What will elute from the column last? -acetylferrocene bc. it's more polar and you switch to the more polar solvent (like dissolves like) What is the absorbent in dry column chromatography? -alumina Why is it imporant the sand is level when packing the column? -there will be portions of the eluent that are not filtered if the sand is slanted What solvents do you use in column chromatography? -petroleum ether (nonpolar) and MtBe (more polar) What is the order of changing test tubes in column chromatograpy? - yellow orange begins to elute, switch, once yellow collected & becomes colorless, switch, change to MtBe, once red orange band has hit bttom switch, until collected, switch Why must the temp. of sand bath be no greater than 60 C? because mp of acetylferrocene is ~80 so may melt