1.LAB 15 – Flashcard
40 test answers
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a) You allowed your chromatogram to develop too long, and you couldn't find the solvent front
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??? If you allowed your chromatogram to develop too long the solutes and solvent would travel too far and possibly run of the plate. Therefore we would not be able to calculate the Rf values without a measured solvent front
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b) The lab assistant who prepared the developing solvent mistakenly used aqueous ammonia in place of acetic acid
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If aqueous ammonia is used in place of acetic acid, ammonia is basic, so that OH group may react with the TLC plate and the acidic samples, and may even form a solid, this the experiment wouldn't work.
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c) You used an open-ended melting-point capillary rather than a Drummond Microcap to apply the spots
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Using an open-ended capillary would cause the spots to be too big that they would run together
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d) You marked the starting line with a ballpoint pen
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Using pen ink would contaminate the plate because it is pigmented. The solvent would drag the ink up the plate because pen ink is soluble in most solvents.
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1. Given aspirin and ibuprofen (shown below) which would you expect to have a lower Rf using a non-polar developing solvent, why?
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a. I would expect Aspirin to have a lower Rf because the solvent is nonpolar and aspirin is more polar than ibuprofen, so the less soluble compound travels less with a lower Rf, while the more soluble travels farther giving a higher Rf
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2. Explain why the Rf value is more informative than the distance traveld by a spot?
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a. The Rf value is more informative than using solely the distance the spot travels because the Rf value allows one to compare different TLC plates, rather than only the compounds measured on one plate. Rf = distance traveled by spot/distance traveled by solvent
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3. What is the highest Rf possible, when would this happen?
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a. The highest possible Rf value is 1. This occurs when the spot travels the same distance as the dissolving solvent.
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4. A pencil should be used to mark the starting line, why is this important?
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a. A pencil should be used rather than a pen, because pen ink can run in most solvents, messing up the results. Pencil will not run.
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5. Why might it be necessary to look at your TLC plate under UV light to see your spots?
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a. It is necessary to look at the TLC plate under UV light because the spots can be colorless, but in UV light the spots can be seen when they quench the fluorescence.
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Lower Rf in a non-polar solvent? Aspirin or ibuprofen
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Lower Rf = less soluble in non-polar = polar Aspirin (Like dissolves like) Polar will not dissolve well in non polar solvent, won't travel far. Aspirin is more polar with O's
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Why is Rf more informative than just distance traveled by spot?
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Rf allows one to compare different plates
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What is highest Rf, when?
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Highest Rf is 1, when unknown compound travels same length of developing solvent
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Why use pencil to mark starting line?
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Pen ink can run in many solvents and will mess up results
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Why use UV light to see spots on TLC plate
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UV allows to see colorless spots when they quench the fluorescence (of the plate)
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Stationary Phase Mobile Phase
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Silica gel in TLC plate Solvent
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More soluble travels greater distance Less soluble travels shorter distance
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Rf = spot distance traveled/solvent distance traveled
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Larger Rf Smaller Rf
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More soluble component Less soluble
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TLC idea
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To separate components based on solubility
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Compounds that quench fluorescence Fluorescent spots are
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Show up as dark spots Bright
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Close lid to developing chamber
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to prevent evaporation from TLC plate saturated with solvent - chamber must be in saturated condition, or plate will not absorb solvent enough
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Capillary Action
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Silica gel sucks up liquid
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Procedure
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1) Spots on starting line 2) TLC plate into Developing chamber of mobile phase solvent and close lid 3) Mobile phase travels up 4) Remove and pencil finish line 5)
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Dissolving Solvent
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ethyl acetate/acetic acid (200:1) 1:1 Ethanol/dichloromethane
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Excedrin BC
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aspirin acetaminophen salicylamide
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Hazardous
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Ethyl acetate - low toxic, doesn't bioaccumulate, Dichloromethane 5ppb in water
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To bind tablet
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Starch
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Antipyretics
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reduce fever (aspirin, acetaminophen, ibuprofen)
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Non steroidal anti inflammatory
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reduce swelling (aspirin ibuprofen)
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Wonder drug
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Aspirin - reduce heart disease, stroke, cancer, cataracts, gallstones
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Prostaglandins and aspirin
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Aspirin blocks over produce of prostaglandins, by turning off key enzyme that makes them (prostaglandin synthase)
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Too many prostaglandins
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blood clots, heart attack, stroke
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Prostaglandin synthase
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assembly line: raw materials in, prostaglandin out
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Aspirin molecules sabotage
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block channel to prevent raw materials entering
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Aspirin
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create cancer fighting substances to prevent digestive cancer
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too much acetaminophen
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liver damage
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ibuprofen leads to
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ulcers, asthma, high blood pressure, kidney, liver, heart disease
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