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Midterm Study Set
Terms in this set (82)
Before coming to lab, you need to
eat food, dress properly, and read and understand experimental procedures and safety precautions.
What is never allowed in lab?
mixing chemicals without specific instruction from lab TA, eating, drinking, smoking, cell phone usage, listening to music
Unauthorized chemistry experiments...
are strictly prohibited.
How do you respond to an emergency situation or accident?
Do not panic. Act quickly. Notify the lab instructor. Seek medical attention.
What should you NOT do if the fire alarm sounds
search for the fire
What is the minimum time required for a chemical exposure to the eyes
What should you locate when entering lab?
eye wash stations, emergency exits, safety shower, and fire extinguisher
What is the minimum amount of time an area of a person's body, that has been exposed to harmful chemicals, should be washed?
will react with water and become flammable or give off flammable or toxic substances
emits ionizing radiation
gas held in an enclosed container at an elevated pressure
readily supplies oxygen to support combustion or oxidation of other substances
may cause a sudden, violent chemical reaction that releases gas and heat
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 actual because the bulb is not exposed to the hot vapor.
True or false? Distillation is a physical separation, not a chemical reaction.
True or false? Over time, the amount of the most volatile component in the distillation flask increases.
True or false? Simple distillation is effective at separating mixtures of liquids with very similar volatilities.
True or false? Over time, the amount of the most volatile component in the distillate increases.
Which distillation technique would provide the best method to separate acetone from decalin.
When the vapor pressure equals the external pressure...
a liquid will boil.
Why is a flask never distilled to dryness?
the flask could crack or break, become charred, or explode because of the residue.
Appropriate glassware for collecting distillate
erlenmeyer flask, round bottom flask, conical vial, graduated cylinder
Inappropriate glassware for collecting distillate
beaker, petri dish, condenser, evaporating dish
After a distillation a student runs an IR of their compound. The student thinks it is pyrrole. Which stretches in the spectrum might lead the student to make this conclusion?
NH stretch at 3550-3250 cm-1
C=C stretch at 1650-1600 cm-1
In what ways can simple distillation be useful in a laboratory setting?
1. To remove dissolved, non-volatile components from a liquid.
2. As the last step in the purification of a liquid.
3. When two components of a system have a boiling point difference of 60 °C or more.
4. To purify more than 60%.
5. To remove solids from a liquid.
What is the minimum boiling point range gap between two solvents that simple distillation can successfully separate?
A 70:30 (mole %) ratio of acetone:water is heated to 43.6 °C at 760 mmHg. What is the total vapor pressure of the system? The vapor pressures for acetone and water at 43.6 °C are 483 mmHg and 74.6 mmHg respectively. and based on the total vapor pressure, what state is it in?
Liquid below boiling
A mixture of acetone and benzene is heated at 67.2 °C and at 760 mmHg. The vapor pressures for acetone and benzene at 67.2 °C are 1090 mmHg and 502 mmHg respectively. Calculate the fractions of acetone and benzene in the liquid phase.
0.44 fraction of acetone in liquid phase
0.56 fraction of benzene in liquid phase
Based on the fractions of acetone and benzene in the liquid phase, calculate the fractions in the vapor phase.
0.62 fraction of acetone in vapor phase
0.37 fraction of benzene in vapor phase
During a fractional distillation of acetone and toluene, a student notices that their data shows a drop in temperature from 56 °C to 51 °C. 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?
1. Pack the column with wire spiral
2. Pack the column with thin metal strips
3. Pack the column with glass helices
4. Pack the column with metal sponge.
Calculate the number of theoretical plates used in a distillation when the column is 2.5 m long, with a HETP (height equivalent to a theoretical plate) of 50 cm.
Pros of using hydrogen as a carrier gas in a GC separation
1. Cheaper than helium
2. Can be generated at the GC instead of from gas tanks
3. Greater difference in mass between carrier gas and sample than nitrogen
Cons of using hydrogen as a carrier gas in a GC separation
It's an explosion hazard
The carrier gas in GC separation is usually....
an inert gas such as nitrogen, helium, or argon, but can also be hydrogen
What procedures(s) can be used to identify a compound in gas chromatography?
1. Enhancing peaks
2. Measuring and analyzing retention times
3. Analyzing with other spectroscopic methods
If the amount of sample injected in GC is doubled, what is the effect on the retention time for the sample?
retention time is unaffected
If the carrier gas flow in GC is increased
retention time decreases
If the column temperature in GC is decreased...
retention time increases
The mobile phase in gas chromatography (GC) is
One type of detector used in gas chromatography is a thermal conductivity detector (TCD). What two elements commonly make up the wire (filament) used in TCDs?
Tungsten and Rhenium
Formula for calculating area between peak........
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?
The melting range would not be affected.
Melting range for a pure substance
narrower than that of an impure sample
Melting range for an impure substance
lower than that of a pure sample
Which of the following statements best describes eutectic composition?
The composition of a mixture of two components that generates the lowest possible melting point.
Which type(s) of molecular interactions determine a molecules melting range?
Hydrogen bonding, dipole-dipole, and 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.
List the seven procedural steps, from start to finish, that are required to perform a thin layer chromatographic separation.
1. Make pencil mark on TLC plate 10 mm from bottom
2. Use capillary to deliver a 2 mm diameter spot onto the pencil mark
3. Add enough developing solvent to cover the bottom 3 mm of the TLC chamber. Cover with a watch glass.
4. Stand the TLC plate upright in the chamber, and replace the lid.
5. Remove TLC plate when solvent is near the top of the plate and mark the solvent front with a pencil before the TLC plate dries.
6. Mark the outline of the spots on the plate of the pencil. Unless spots are colored, a visualization method light will be needed
7. Measure the distance that the spot and the solvent have moved
In TLC, if you have two compounds, what can be said about the more polar substance?
It travels the shorter distance
Name 3 things about the principles of thin-layer chromatography?
1. If the concentration of the component is too high, the spot will become a smear or begin to tail.
2. Separation will be achieved if one component adheres to the stationary phase more than the other component does.
3. The solvent moves by capillary action.
A TLC plate was run in 1:1, hexane: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?
spot a new plate and run in 1:4, hexane:acetone
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
Calculate Rf value
1. Measure from starting point to dot
2. Measure from starting point to solvent front
Divide #1/ #2.
separation of substances
physically blocking certain substances while letting other sthrough
pouring off a liquid but keeping the solid
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.
Which is 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
Good extraction systems
1. water + dichloromethane
2. water + diethyl ehter
Poor extraction system
1. diethyl ether + dichloromethane
2. water + ethanol
3. acetone + diethyl ether
If there is 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(aq)?
A reaction 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(aq), followed by removal of the water layer. The organic layer then undergoes complete treatment with HCl(aq). 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 (DCM) 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.
A student separates a 2.50 g sample of an unknown mixture of benzoic acid, 4-nitroaniline, and benzil using an acid-base extraction procedure. After the separation, the student finds that there is 0.780 g of benzoic acid, 0.650 g of 4-nitroaniline, and 1.17 g of benzil. Calculate the total percent recovery (include three significant figures).
What mistakes during the extraction procedure could lead to a percent recovery above 100%?
Not all of the drying reagent was filtered out.
The samples were not dried enough.
Magnesium sulfate, sodium sulfate, calcium sulfate, calcium chloride, potassium carbonate, brine (NaCl)
Not a drying agent:
glucose, graphite, chlorine, arginine
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 clumped together at the bottom of the flask. What does this observation indicate?
Not enough drying agent has been added.
Assume an organic compound has a partition coefficient between water and ethyl acetate equal to 6.38. If there are initially 6.79 grams of the compound dissolved in 70.0 mL of water, how many grams will remain in the aqueous layer after extraction with one 15.0 mL portion of ethyl acetate?
Assume an organic compound has a partition coefficient between water and ethyl acetate equal to 7.53. If there are initially 6.09 grams of the compound dissolved in 100.0 mL of water, how many grams will remain in the aqueous layer after extraction with two 15.0 mL portions of ethyl acetate?
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 °C, 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?
Rank by reactivity: amides, acid anhydrides, esters, acid chlorides
acid chlorides > acid anhydrides > esters > amines
What are the products of the methyl salicylate hydrolysis reaction shown below? (Select all that apply)
2. salicylic acid
As time progresses during the an esterification reaction, the m-tolyl acetate spot on a TLC plate will become
THIS SET IS OFTEN IN FOLDERS WITH...
Simple & Fractional Distillation
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