Question 1

Infrared Spectra are recorded using an ATR detector. What does ATR stand for?

Accepted Answer

Attenuated Total Reflectance

Question 2

In which of the following ways is the use of an infrared spectrophotometer fitted with an ATR sample advantageous over a beam-through-sample spectrophotometer? Select all answers that apply.

Accepted Answer

ATR allows for the use very small samples.  The same ATR sample holder may be used for liquid samples and solid samples.

Question 3

What region of an infrared spectrum is most useful for distinguishing between isomers that contain the same functional group?

Accepted Answer

1400-400 cm-1

Question 4

Which of the following statements about infrared spectroscopy is not true?  A. Infrared spectroscopy can detect impurities within a sample.  B. Infrared spectroscopy is non-destructive.  C. Infrared spectroscopy can be used to analyze almost any kind of sample.  D. Infrared spectroscopy is good for detecting and identifying functional groups of a sample.

Accepted Answer

C. Infrared spectroscopy can be used to analyze almost any kind of sample.

Question 5

Rank the following molecules in terms of their carbonyl stretching frequency, ?(C=O), in the infrared spectrum.  benzaldehyde  pentanal  cinnamaldehyde

Accepted Answer

pentanal  benzaldehyde  cinnamaldehyde

Question 6

How would the melting range of a pure organic material be changed if there was a tiny shard of broken glass inside the melting point capillary?

Accepted Answer

The melting range would not be affected.

Question 7

Which of the following statements best describes a eutectic?

Accepted Answer

The composition of a mixture of two components that generates the lowest possible melting point.

Question 8

Why is it good practice to perform a mixed melting point determination at two different ratios of unknown to known materials in the melting point capillary?

Accepted Answer

Because it is possible that one particular combination of materials generates a eutectic.

Question 9

Which of the following statements about melting point determination are true? Select all that apply.

Accepted Answer

The lower limit of the melting temperature range occurs when the first crystal starts to melt.  Pure substances typically have a melting range of less than 2 degrees Celsius.  The upper limit of the melting temperature range occurs when the last remaining crystal melts.

Question 10

When comparing melting temperature ranges of pure and impure samples, which of the following statements describes the melting temperature range of an impure sample? Select all answers that apply.

Accepted Answer

The melting temperature range of a pure sample is narrower than that of an impure sample.  The melting temperature range of an impure sample is lower than that of a pure sample.

Question 11

The retention factor, or Rf, is a number that describes the position of a spot on a chromatography plate relative to the position of the solvent on the chromatography plate. What are the minimum and maximum values of Rf?

Accepted Answer

0-1

Question 12

What error is introduced if the bulb of the thermometer, or the thermocouple, is too high in the distillation apparatus?

Accepted Answer

The recorded boiling point will be lower than actual because the bulb is not exposed to the hot vapor.

Question 13

Which of the following statements describe why moisture must be excluded from any sample injected into the G.C. Select all answers that apply.

Accepted Answer

Water contains dissolved salts that can degrade the column.  Water signals are very broad and make interpretation of other peaks difficult.

Question 14

Which of the following statements about the collection of a G.C. trace is true. Select all that apply.

Accepted Answer

The amount of sample detected is proportional to the peak area.  The stationary phase is a very high-boiling liquid adsorbed onto fire brick.

Question 15

Which of the following experimental errors would lead to the observation of peaks trailing in on the G.C. trace?

Accepted Answer

The oven temperature is too low.  The column is overloaded.

Question 16

Which of the following experimental errors would lead to the observation of peaks trailing off on the G.C. trace?

Accepted Answer

The sample is too concentrated.  The column is overloaded.

Question 17

For a packed column, doubling the length essentially doubles the number of theoretical plates. This will increase the effectiveness of the separation by the square root of 2. In order to double the separation, one would need to increase the column length _______.

Accepted Answer

four times

Question 18

A two-component mixture of volatile liquids was separated by gas chromatography using a 5 meter capillary column. The separation was insufficient with this column to see resolution of both fractions. Without changing any of the other parameters, what length of column must be used to double the efficiency of this chromatographic separation?

Accepted Answer

20 meters

Question 19

There are four major methods of purification in the chemistry laboratory. Which of the following techniques are included in the four most common methods for separation and purification? Select all that apply.

Accepted Answer

Extraction  Recrystallization  Distillation

Question 20

When is recrystallization not a suitable method to purify an organic material?

Accepted Answer

When the organic material is not a solid.

Question 21

The solubility of acetanilide is 3.53 g in 100 mL of chloroform at 0 Â°C, and 29.1 g in 100 mL of chloroform at 60 Â°C. What is the maximum percent recovery that can be achieved for the recrystallization of acetanilide from chloroform?

Accepted Answer

87.9%

Question 22

A student was given a sample of crude acetanilide to recrystallize. The initial mass of the the crude acetanilide was 146 mg. The mass after recrystallization was 123 mg. Calculate the percent recovery from recrystallization.

Accepted Answer

84.2%

Question 23

Which of the following statements best describes density?

Accepted Answer

The ratio of a substance\'s mass to its volume, measured in g/mL

Question 24

During a solvent extraction, a student lost track of which layer was the aqueous layer and which was the organic layer. What simple, non-destructive test could be used to determine which layer is which?

Accepted Answer

Add a little distilled water to see which layer the water adds to.

Question 25

Eugenol, the essential oil found in cloves, is insoluble in water. The alcohol group in eugenol is dramatically more acidic than regular straight-chain alcohols. How could eugenol be separated from a mixture of other non-acidic molecules?

Accepted Answer

Dissolve the mixture in dichloromethane, shake with sodium hydroxide, collect the aqueous layer, and reacidify to force precipitation of the eugenol.

Question 26

Convert the benzocaine into a water-soluble salt.

Accepted Answer

add acid to the three component mixture

Question 27

To avoid a build up of pressure inside the glassware.

Accepted Answer

vent the separatory funnel through the stopcock

Question 28

To precipitate the benzocaine.

Accepted Answer

add base to the aqueous acidic layer mixture

Question 29

To avoid spilling chemicals on others or yourself.

Accepted Answer

never point the seperatory funnel at anyone, including yourself

Question 30

How is it possible to separate two liquids with exactly same boiling point by gas chromatography (G.C.) when it cannot be done by fractional distillation?

Accepted Answer

The use of a polar GC column will separate the liquids based on differences in polar interactions and volatility.

Question 31

Less than 1 mL of the sample is available  The components of the liquid mixture are not thermally stable  The components of the liquid mixture have identical boiling points  Only a tiny amount of a pure sample is needed

Accepted Answer

Preparative GC

Question 32

Several liters of the sample need to be separated

Accepted Answer

Fractional Distillation

Question 33

The carrier gas in a G.C. separation is usually an inert gas such as nitrogen, helium, or argon, but can also be hydrogen. What are the pros and cons of using hydrogen as a carrier gas in a G.C. separation?

Accepted Answer

Pros:  Cheaper than helium  Can be generated at the GC instead of from gas tanks  Greater difference in mass between the carrier gas and sample than nitrogen  Cons:  Explosion hazard

Question 34

What physical properties prevent a sample mixture from being analyzed by gas chromatography (GC)? Select all that apply.

Accepted Answer

One or more of the components is not volatile.  One or more of the components is thermally unstable.

Question 35

A 0.7 mL mixture of pentanol and heptanol are separated and collected using preparative gas chromatography on a GC fitted with a polar column and a TCD detector. During the separation, why are the injection port and detector heated to a higher temperature than the column? Select all that apply.

Accepted Answer

The wire in the thermal conductivity detector needs to be cooled by the eluent in order to register a signal.  A heated injector creates flash vaporization which allows separation at temperatures lower than the boiling point of the sample.

Question 36

What effect would each of the following changes to the conditions of a GC separation have on the retention time for a sample. Use the pop-up menus to identify the change  The column temperature is increased.  The carrier gas flow is decreased.  The amount of sample injected is doubled.

Accepted Answer

Decreases  Increases  Increases

Question 37

To avoid the introduction of a spurious peak into the GC trace.

Accepted Answer

Do not rinse the syringe with acetone, rinse it with the solution instead

Question 38

To avoid dangerous vibration due to imbalance.

Accepted Answer

Vials are placed into the centrifuge in pairs, 180 degrees apart

Question 39

To avoid loss of sample.

Accepted Answer

The detector fitted to the Prep GC is a TCD not a FID

Question 40

To preserve the purity of the sample because there is a lag between being detected and being eluted.

Accepted Answer

Wait for the second peak to be on the rise before beginning its collection

Question 41

A student is comparing two samples of crystalline organic solids. A TLC analysis shows a single spot for each sample and the co-spot with the same Rf value. Which of the following would provide the most conclusive evidence that the two samples are the same compound?

Accepted Answer

Perform a mixture melting point.

Question 42

A TLC plate was run in 1:1 hexane and acetone, but when developed the plate only showed baseline spots. What change could be made to get the spot(s) to move further up the plate?

Accepted Answer

Spot a new plate and run in 1:4 hexane/acetone

Question 43

T/F  Silica is toxic.

Accepted Answer

False

Question 44

T/F  Hexane is flammable.

Accepted Answer

True

Question 45

T/F  Ethyl acetate is flammable.

Accepted Answer

True

Question 46

T/F  Acetone is flammable.

Accepted Answer

True

Question 47

T/F  It is acceptable to handle TLC plates with bare hands.

Accepted Answer

False

Question 48

What is the major underlying principle of chromatography? (including TLC)

Accepted Answer

Separation will be achieved if one component adheres to the stationary phase more than the other component does.

Question 49

Why is there usually a piece of filter paper in the TLC developing chamber?

Accepted Answer

To create an atmosphere saturated with developing solvent.

Question 50

For a successful TLC experiment, you need to pay attention to the details. From the list of instructions and tips below, select the correct itmes.

Accepted Answer

Use only a plain pencil to draw a starting line and mark the compounds\' spots. Never use a pen or marker!  Cover the developing chamber with a watch glass.  Do not permit the solvent to rise to the very top of the TLC plate.  Do not disturb the developing chamber, do not move it or carry around.  Once the solvent front is getting close to the top of the plate, remove the plate and mark the solvent front.  The solvent level in the chamber should be lower than the starting line where you spot the samples.  Use a micropipet or glass capillar to put a small amount of the sample solution on the starting line.

Question 51

Acetyl coenzyme A precursor  Acetogenin

Accepted Answer

Eugenol

Question 52

Isopentyl diphosphate precursor  Terpene

Accepted Answer

Limonene

Question 53

Tests positive for bromine and potassium permanganate tests?

Accepted Answer

cyclohexene  limonene

Question 54

Tests positive for bromine, potassium permanganate, and ferric chloride tests?

Accepted Answer

Eugenol

Question 55

Both Br2 and HCl undergo electrophilic addition to alkenes. Why is Br2 used as a test for unsaturation instead of HCl?

Accepted Answer

The reaction with bromine can be followed because of a visible loss of color.

Question 56

The ether product is colorless.

Accepted Answer

Ultraviolet light was shone on the TLC plates

Question 57

Detergents are soluble in both organic and aqueous phases.

Accepted Answer

Benzyltributylammonium chloride was added to the reaction mix

Question 58

Dichloromethane solvent is highly volatile.

Accepted Answer

The reaction vial was stoppered before it was stirred

Question 59

To avoid making the reaction mix become an emulsion.

Accepted Answer

The reaction was stirred at a moderate, not rapid, pace.

Question 60

The non-polar ether product can be separated from the polar by-products.

Accepted Answer

The contents of the reaction flask were passed through a column of silica.

Question 61

The Williamson ether synthesis between 2-naphthol and allyl bromide in the presence of strong base and benzyltriphenylammonium chloride as a phase transfer catalyst in dichloromethane solution is known to proceed in reasonable yield. Why will the reaction fail to work if the solvent (NAOH) is switched to ethanol while retaining the same reactants, phase transfer catalyst, and base?

Accepted Answer

Because the base would deprotonate ethanol and create a second nucleophile.

Question 62

The Williamson ether synthesis between 2-naphthol and allyl bromide in the presence of strong base and benzyltriphenylammonium chloride as a phase transfer catalyst in dichloromethane solution is known to proceed in reasonable yield. Why will the reaction fail to work if the reactants (2-naphthol and allyl bromide) are switched to 2-bromonaphthalene and allyl alcohol while retaining the same phase transfer catalyst, solvent, and base?

Accepted Answer

Because the bromo group is a poor leaving group when attached to an aromatic ring.

Question 63

In the reaction to produce allyl 2-naphthyl ether, 2-naphthol is first reacted with aqueous sodium hydroxide to make sodium 2-naphtholate, which then goes on to react with allyl bromide and benzyltributylammonium chloride phase transfer catalyst, with dichloromethane as a solvent. What is in the aqueous (top) layer?

Accepted Answer

Water  sodium 2-naphtholate  sodium hydroxide   benzytributylammonium chloride

Question 64

In the reaction to produce allyl 2-naphthyl ether, 2-naphthol is first reacted with aqueous sodium hydroxide to make sodium 2-naphtholate, which then goes on to react with allyl bromide and benzyltributylammonium chloride phase transfer catalyst, with dichloromethane as a solvent. What is in the organic (bottom) layer

Accepted Answer

Dichloromethane  allyl bromide  allyl 2-naphthyl ether  benzyltributylammonium chloride

CHEM 3238 lab 1 – Flashcards

Unlock all answers in this set

Haven't found what you were looking for?

Search for samples, answers to your questions and flashcards