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OChem Lab Test 1

Terms in this set (73)

In a typical IR spectrum, what name describes the units shown on the x-axis?
Wavenumber
In a typical IR spectrum, what name describes the units shown on the y-axis?
% Transmittance
What type of electromagnetic radiation is associated with the peaks in the IR spectrum?
Infrared radiation
What is measured in the IR method?
the vibrational frequency of a bond
What type of information is given in IR spectroscopy?
the functional groups in a molecule
What wavenumber do stronger bonds absorb at?
Stronger bonds absorb at a higher wavenumber
In infrared spectroscopy, what must chemically occur within a compound to have a signal appear in the IR spectrum?
dipole moment
Wavelength
the distance between two crests of an electromagnetic wave
Frequency
the rate at which electromagnetic waves oscillate
What are the units of wavenumbers in IR spectroscopy?
Inverse centimeters (Cm^-1)
What region of an infrared spectrum is most useful for distinguishing between isomers that contain the same functional group?
1400-400 cm^-1
The IR spectrum of a compound shows several -weak bands from 1750-1950 cm^-1. What is the correct origin of these bands?
aromatic overtone-combination bands
One technique for taking the IR spectrum of a solid sample is by making a mull. A mull is made by mixing Nujol, a heavy paraffin oil, and the solid sample. The mull is then placed between two salt plates and the IR spectrum measured. Select the statement that is true about a mull.
When the solid sample is mixed with the Nujol, a mull is formed and has a thick consistency like that of toothpaste. IR signals from Nujol can be subtracted out by measuring a blank of pure Nujol.
Which of the following statements apply to the operation of an IR spectrometer?
The sample plates must never be handled with bare hands & the sample plates are fragile and will crack or chip if mishandled.
A region of the IR spectrum can be used to differentiate between similar compounds that contain the same functional groups. Select the correct name of this region.
the fingerprint region
in Fourier Transform Infrared (FTIR) spectroscopy, which type requires the most energy?
stretching
Which peak is assigned a relative intensity of 100?
base peak
Which peak is the most intense peak in the mass spectrum?
base peak
Which peak represents the original molecule that has only lost an electron?
molecular ion peak
What could be/represnet a smaller charged fragment of the original molecule?
the base peak & a radical cation
What is/represents a positively charged species?
the base peak, the molecular ion, a radical cation, and the parent ion
Benzene is studied via electron ionization mass spectrometry. Select the most likely m/z value for the molecular ion peak of benzene.
78
What error is introduced if the bulb of the thermometer, or the thermocouple, is too high in the distillation apparatus?
the recorded boiling point will be lower than the actual because the bulb is not exposed to the hot vapor
Distillation is a physical separation, not a chemical reaction.
True
Over time, the amount of the most volatile component in the distillation flask increases
False
Simple distillation is effective at separating mixtures of liquids with very similar volatilities
Flase
Over time, the amount of the most volatile component in the distillate increases
True
A liquid will boil when...
the vapor pressure equals the external pressure
Why is a flask never distilled to dryness?
the flask could crack or break, the flask could become charred, and the flask could explode because of residue
What kind of glassware is appropriate for collecting distillate?
Erlenmeyer flask
round bottom flask
conical vial
graduated cylinder
What kind of glassware is inappropriate for collecting distillate?
beaker
petri dish
condenser
evaporating dish
In what way(s) can simple distillation be useful in a laboratory setting?
when two components of a system have a boiling point difference of 60 degrees Celsius or more
to remove dissolved, non-volatile components from a liquid
to remove solids from a liquid
to purify more than 60%
as the last step in the purification of a liquid
What is the minimum boiling point range gap between two solvents that simple distillation can successfully separate?
60-70 degrees Celsius
During a fractional distillation of acetone and toluene, a student notices that their data shows a drop in temperature from 56 to 51 degrees Celsius. What does the student conclude?
There is no more acetone vapor and the toluene has not started to distill.
How can the efficiency of fractionating columns be improved?
pack the column with metal sponge
pack the column with glass helices
pack the column with thin metal strips
pack the column with wire spiral
The carrier gas in GC 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 GC separation?
Pros:
cheaper than helium
can be generated at the GC instead of from gas tanks
greater difference in mass between carrier gas and sample than nitrogen

Cons:
explosion hazard
Which procedure(s) can be used to identify a compound in gas chromatography?
enhancing peaks,
analyzing with other spectroscopic methods,
measuring and analyzing retention times
How can the injection time be indicated on a gas chromatograph?
if the detector is a thermal conductivity detector, inject several microliters of air with the sample

if the GC has analysis software, it will automatically be shown on the print out

use a pen to mark on the recorder base line immediately after the injection
The stationary phase in gas chromatography (GC) is
liquid
One type of detector used in gas chromatography is thermal conductivity detector (TCD). What two elements commonly make up the wire (filament) used in TCDs?
Tungsten and Rhenium
Which of the following statements about melting point determination is not true?
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

The lower limit of the melting temperature range occurs when the first crystal starts to melt

The melting point is a physical measure of the forces holding the material into its crystal lattice
What effect do impurities have on the melting point of a substance?
Impurities lower the melting point
Which of the following statements best describes eutectic composition?
The composition of a mixture of two components that generates the lowest possible melting point
When comparing melting temperature ranges of pure and impure samples, which of the following statements describes the melting temperature range of an impure sample?
The melting temperature range of an impure sample is broader than that of a eutectic

The melting temperature range of an impure sample is higher than that of a eutectic
Which type(s) of molecular interactions determine a molecules melting range?
Hydrogen bonding
Dipole-dipole
Van der Waals
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?
Perform a mixture melting point
In TLC, if you have two compounds, what can be said about the less polar substance?
it travels the longer distance
Which statements are true about the principles of thin-layer chromatography?
If the concentration of the component is too high, the spot will become a smear or begin to fail

The solvents moves by capillary action

Separation will be achieved if one component adheres to the stationary phase more than the other component does
A TLC plate was run in 1:1, hexane:ethyl acetate, but when developed the plate only showed baseline spots. What change can be made to get the spot(s) to move further up the plate?
spot a new plate and run in 1:4, hexane:ethyl acetate
A student develops their TLC plate and places it under a UV light, but nothing appears. What mistake might the student have made?
spotted the compound below the level of eluent
Extraction
separation of the substances
Filtration
physically blocking certain objects or substances while letting others through
Decantation
pouring off a liquid but keeping the solid
Sublimation
when a solid turns into a gas without turning into a liquid
What is extraction>
the process of transferring a solute from one solvent into another solvent
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?
Add a little distilled water to see which layer the water adds to
A layer will form between the aqueous and organic phases if the extraction is shaken too vigorously. What is the term to describe this layer?
emulsion
Which of the following extraction regiments is the most effective for extracting a mixture of compounds X and Y with ether?
three extractions of 25 mL of ether each
Which pair of solvents would make good extraction systems?
water + dichloromethane and water + diethyl ether
In an aqueous workup of a reaction, determine which solvent is located in the separatory funnel.
The top layer consists of the organic phase, which is diethyl ether in this experiment.

The bottom layer consists of the aqueous phase, which is water.
If there were a reaction mixture in an organic solvent that contains an organic acid, an organic base, neutral organic compounds, and base containing impurities, what is in the aqueous layer after complete treatment with NaOH?
organic acid
A reacting mixture, in an organic solvent, contains an organic acid, an organic base, neutral organic compounds, and base containing impurities. The organic solvent is treated completely with NaOH, followed by the removal of the water layer. The organic layer then undergoes complete treatment with HCl. What is left in the organic layer after this treatment.
neutral organic compounds
At what point during the process of extraction is a "drying agent" used?
before the solvent is evaporated to yield the final product
An organic compound was extracted into dichloromethane and then the aqueous layer is shaken with saturated sodium chloride solution. What is the purpose of the sodium chloride?
to decrease the solubility of the organic product in water
What mistakes during the extraction procedure could lead to a percent recovery above 100%?
the samples were not dried enough and not all of the drying reagent was filtered out
What are good drying reagents?
magnesium sulfate
sodium sulfate
calcium sulfate
calcium chloride
potassium carbonate
brine
A reaction was performed, and the dichloromethane solvent was dried by adding magnesium sulfate drying agent. When the reaction flask was shaken, it was observed that the magnesium sulfate swirled around freely at the bottom of the flask. What does this observation indicate?
the correct amount of drying reagent has been used
During lab, a closed system should not be heated because
the setup could explode
A water bath should be used in which of the following situations?
keeping a flask or test tube containing a chemical substance at a specific, constant temperature
While heating a vessel at approximately 65 degrees Celsius, which contains a reaction using a hexanes and water solution, a student notices the volume in the vessel has significantly decreased. What should the student add to the reaction to continue the reaction properly?
hexanes
What is the purpose of DMAP in the esterification reaction?
catalyst
Why must cyclopentadiene be freshly distilled and kept cold prior to a Diels-Alder reaction?
cyclopentadiene dimerizes at room temperature
Which of the following will facilitate a normal Diels-Alder reaction?
an electron withdrawing group on the dienophile
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