Chem Lab Final
What is the solid phase in thin-layer chromatography?
TLC PLATE
What is an electron-rich or electron-donating species called?
nucleophile
What is the product of a Grignard reaction?
A 2° or 3° alcohol with the α-carbon bound to the R group from the Grignard reagent.
Which of the following is NOT a component of a TLC system?
Heating filament
Which of the following BEST describes what the blue arrow is pointing to?
This is the stationary phase in column chromatography.
What is an electron-poor or electron-withdrawing species called?
electrophile
Which of the following statements about immiscible mixtures is TRUE?
The boiling point of the mixture is lower than the boiling point of any of the components.
When setting up the distillation setup, which port on the condenser serves as the water inlet?
The port furthest from the round-bottom flask.
Vapor pressure is defined as:__?
The pressure exerted by the gaseous phase in contact with the liquid or solid phase of a substance in a closed system
The setup in the image represents which of the following methods?
Vacuum filtration
Uneven flow through a column...
can cause poor separation of compounds.
In the distillation setup shown, what is the "distillate"?
iv
A chemist measured the melting point of an organic compound before and after she recrystallized it to remove impurities. Which of the following is a plausible value for the reported melting point in literature?
114 °C
Which of the following is NOT a property that can be used to separate and purify mixtures?
Hygroscopicity
What is important to consider when choosing solvents?
Solvent and solute polarity
Which of the following is NOT true of reverse-phase HPLC?
Solvents are never degassed to ensure that there are a lot of gas bubbles in the mobile phase.
Which of the following statements regarding acid-base extractions is NOT correct?
The addition of an acid or base disrupts the organic phase, allowing the organic compound to become more soluble in water.
Which of the following statements regarding the partition coefficient is correct?
The higher the partition coefficient of a solute, the less likely it is to partition into the aqueous phase.
What is the definition of vapor pressure?
The total pressure of a vapor in equilibrium with its condensed phase.
How can you monitor the progress of a Grignard reaction?
Use TLC to monitor the consumption of the carbonyl-containing compound
What is the mobile phase in gas chromatography?
Vaporized sample and inert carrier gas.
A student needs to remove impurities from an old batch of an organic compound. According to the table, which solvent is ideal for recrystallization of the compound?
Water
How are components separated using distillation?
When heated, the substance with the lower boiling point is vaporized first, then it is condensed and collected.
At what point is it time to end a distillation procedure?
When there is a thin film of liquid left in the round-bottom flask.
Solvents that interact WEAKLY with the stationary phase...
are weak eluents.
For good separation, the mobile phase should be...
as nonpolar as possible.
Rf is the characteristic ratio of the distance a compound traveled up a TLC plate to...
the distance the solvent traveled up the plate.
What should you avoid allowing into the reaction mixture when preparing and using a Grignard reagent?
water
The following image displays the boiling points of compounds i, ii, and iii in solution. The thermometer represents the current temperature of the solution.
Compound i is the most volatile component of the solution.
The fluid that is collected at the bottom of this setup is called a(n)
Filtrate
How full should the round-bottomed flask be?
Less than half full
Which of the following statements regarding solvent pairs is NOT correct? The density of hexane is 0.659 g/mL and the density of DMF is 0.944 g/mL.
A mixture of toluene and DMF will form a biphasic system with toluene on top and DMF on the bottom.
Which of the following statements about the solvent in a TLC setup is/are correct? i. Two solvents are sometimes combined to achieve the desired polarity ii. The development solvent is usually less polar than the stationary phase.
Both (i) and (ii)
What allows the mobile phase to move up the TLC plate?
Capillary action
You are a laboratory technician interning at a major forensic science institution. You are tasked with separating an unknown mixture containing different organic compounds. Your supervisor has mentioned that the components in this mixture seem to be immiscible in water. The best way to separate these organic compounds is by:
Hydrophobicity
Under which condition can we separate impurities before recrystallizing the compound to be purified?
If the impurities are insoluble in the hot solvent.
The formation of the first crystals during recrystallization is called
Nucleation
What is the first step of crystallization?
Nucleation
Which of the following statements is NOT CORRECT about extractions?
Only liquid phase solutes can be extracted.
Why must the FID always be hotter than the GC column?
Preventing condensation.
Two students are attempting to separate the dyes found in a purple food dye. They both prepare the silica gel columns using the same materials and the same procedure. They both load sample on to the top of the column and layer the solvent on top. The image shows the separation of the dyes, Student A on the left, and Student B on the right. Which of the following statements provides the BEST explanation for the observed results?
Student B added more of the sample than Student A.
A student assembles a silica gel column to separate green dye, following the same procedure outlined in this lab. As the dye runs the column, the student notices streaking lines of dye being left behind as the band moves down the column. Which of the following statements is correct about the phenomenon observed?
The stationary phase was not properly packed. A channel is present in the stationary phase. Correct! All of these statements are correct. Air bubbles or dry patches are present in the column.
A student synthesized benzoic acid by reacting sodium benzoate and hydrochloric acid in deionized water and vacuum filtered his product. Why did he pour the remaining deionized water over the benzoic acid solids on filter paper inside the Büchner funnel?
To dissolve and remove any remaining sodium benzoate.
Retention time is...
how long the analyte is in the column.
What can you use to prevent peak broadening of less polar compounds and improve peak separation?
mobile phase gradient
When does boiling occur?i. When the energy gained by a liquid overcomes the intermolecular interactions in the liquid phase.ii. When the total vapor pressure of the substance is equal to the atmospheric pressure.iii. When the total vapor pressure of the substance is less than the atmospheric pressure.
(i) and (ii)
Which impurities tend to separate in a recrystallization experiment?i. Impurities that are insoluble in the solvent at high temperaturesii. Impurities that are soluble in the solvent at room temperature
(i) and (ii)
Which of the following methods can be used to detect the solute spots on the TLC plate? i. Putting the plate under a UV lamp ii. Placing the plate inside a jar with iodine iii. Feeling the deposited crystals on the plate with fingertips
(i) and (ii)
Which of the following statements about the bubble point is correct?i. The bubble point is the temperature at which the first bubble of vapor starts to form.ii. For a pure liquid, the boiling point and bubble point are the same.iii. For a miscible mixture, the boiling point of the mixture and the bubble points of each component are the same.
(i) and (ii)
Which statements about crystallization are correct?(i) Crystallization can happen spontaneously.(ii) Providing a growth surface encourages crystallization.(iii) Rate of cooling does not affect crystallization.
(i) and (ii)
Which statements about the retardation factor values in the table are CORRECT? i. Compound A is more polar than compound B ii. Compound C is less polar than compound D iii. Compound C is less attracted to the stationary phase than compound B.
(i) and (ii)
Which of the following statements must be true about the solutes found in the hot water of this brewing teacup? i. These solutes are water-soluble. ii. These solutes must be essential oils from the tea leaves. iii. These solutes have a high solubility in aqueous solutions.
(i) and (iii)
Which of the following statements about miscible liquids is correct?i. The components form a homogeneous solution.ii. The partial pressure of each component is the vapor pressure of the mixture times the components mole fraction.iii. Each component has its own vapor pressure.
(i), (ii) and (iii)
Which of the following glassware is the MOST appropriate to perform a liquid-liquid extraction?
(ii)
A student prepares the following solvent-pairs, labeled (i)-(iv). Which of these are appropriate for liquid-liquid extraction? (i) Acetone mixed with dimethylformamide (ii) Dimethyl sulfoxide mixed with hexane (iii) Toluene mixed with water (iv) Benzene mixed with hexane
(ii) and (iii)
A student has assembled the following steam distillation apparatus. Which of these statements is CORRECT about this assembly? i. The extraction flask is in the wrong place and will not replenish water into the system. ii. The thermometer is not placed in the correct spot for steam distillation. iii. The wrong heat source is being used. iv. The wrong condenser is being used.
(iii) only
What is the retardation factor of the organic compound in this example?
0.67
If the partition coefficient of benzoic acid in a benzene-water biphasic system is 0.636, how much benzoic acid is present in the aqueous phase if a 7.5 M benzoic acid solution has mixed at a 1:1 ratio with benzene?
4.58 M
Preparatory HPLC is used to...
purify compounds.
How can you avoid air bubbles creating voids in the stationary phase and destabilizing the detector readings?
Always degas solvents and carbonated samples.
Which of the following methods is the BEST way to separate mixtures on a microscale?
Column chromatography
Compound A is more volatile than Compound B. Which of the following statements about the relationship between A and B is CORRECT?
Compound A has a higher vapor pressure than Compound B.
Which labware are used to assemble a gravity filtration setup?
Conical funnel, filter paper, Erlenmeyer flask
Which of the following calculations determines the percent recovery?
Dividing the mass of recrystallized product by the mass of starting materials and multiplying the result by 100.
What is the primary use of a rotary evaporator?
Drying and purifying samples
Essential oils in fruits and plants provide the familiar scents associated with them. As volatile organic compounds, they readily enter the gaseous phase at room temperature. Which of the following statements about volatile organic compounds is TRUE?
Fractional distillation cannot be used to extract volatile organic compounds because high temperatures may decompose or change the compound into something else.
Which of the following will not encourage crystallization?
Further increasing the temperature of the solution
What is in a fractionating column?
Glass or plastic beads.
How does the composition of the distillate change over the course of a distillation?
It begins rich with the more volatile phase, and ends rich with the less volatile phase.
What is the purpose of the highlighted equipment in the steam distillation setup?
It is a water reservoir.
Which of the following statements about a silica gel-based stationary phase is CORRECT?
It is extremely polar.
How is the retardation factor of an organic compound calculated in TLC?
It is the distance traveled by the solute divided by the distance traveled by the mobile phase.
Why is the water reservoir important in steam distillation?
It lowers the boiling temperature of the mixture, allowing for extraction of organic compounds below their boiling point.
What should NOT be considered when trying to prevent bumping?
Magnetic fields
In a TLC setup, the distance a solute molecule travels on the stationary phase depends mainly on its ______________ .
Polarity
What is recrystallization used for?
Purifying solid compounds.
Which statement about the rate of cooling during recrystallization is correct?
Rapid cooling allows more nucleation sites and smaller crystals.
A scientist wants to purify compound (i) from a mixture that she suspects to be contaminated by compounds (ii) and (iii). She adds 100 mL of boiling water to 0.4 g of mixture. Which of the following is the correct sequence of crystallization (Water solubility characteristics of each compound are listed in the table)?
Remove (iii) with hot filtering then remove (i) by filtering at room temperature.
Why does scratching the bottom of a flask encourage crystallization?
Scratching the bottom produces a non-smooth surface, ideal for crystal nuclei to form.
One popular product that is used in water purification is a filtration straw. The straw passively removes particulates from contaminated water as the consumer sucks up water. What property is this product MOST LIKELY using to remove particulates from contaminated water?
Size
How should you start putting the system under vacuum?
Start with a low vacuum and increase the vacuum strength as needed until solvent begins condensing in the trap or the solution starts bubbling.
Two students prepare a silica gel column to separate the dyes found in a permanent marker. The dyes found in these markers have similar hydrophobicity. Student A prepares the column on the left. Student B prepares the column on the right. Which of the following is the MOST LIKELY experimental outcome?
Student B's fractions have a lower purity than Student A's fractions.
Which of the following statements is TRUE about a fast-moving band moving through a chromatography column from a heterogenous mixture?
The band has the weakest interactions with the stationary phase.
When a mixture first reaches its boiling point, what component transitions to vapor first?
The component with the lower boiling point.
In the boiling point diagram shown, what does the top dashed line represent?
The dew point
A student prepares a solvent of benzene and water for use in liquid-liquid extraction of a branched long-chain fatty acid in an aqueous solution. Based on the table and the supplied image, which phase should the student move forward with?
The student should move forward with the top blue phase as this is the organic phase that contains her product.
What is the definition of the dew point?
The temperature at which vapor starts to condense.
Which of these is NOT required for a two-solvent system?
The two solvents are an acid-base pair.
What is the definition of partial pressure?
The vapor pressure of one component in a mixture.
What happens to the vapor in a fractionating column?
The vapor repeatedly condenses on and evaporates from the beads, allowing more volatile liquids to reach the top of the column faster.
Which of the following statements BEST describes the liquid in the highlighted area from a steam distillation of essential oils from a fruit?
This liquid requires further extraction to isolate the essential oils.
What should you do before baking the column and setting up the autosampler?
Turn on the flow of inert carrier gas.
Why is it important to vent the separatory flask after every mixing step?
Venting prevents a build-up of vapor pressure caused by agitation of the organic solvent.
The dimensions of an FID peak are proportional to the...
mass of carbon in the analyte corresponding to that peak.
A student wants to separate a mixture of two compounds into its components based on their differences in solubility in water. Both compound A and compound B dissolve in boiling water. Compound A starts recrystallizing at 82 °C, whereas compound B recrystallizes around 35 °C. Which experimental time point is ideal to separate the compounds?
Filter at (ii) using 50°C water
How do you prepare a Grignard reagent?
React an organohalide with magnesium metal.
What occurs in an SN1 reaction?
Slow departure of the leaving group, then nucleophilic attack on either side of the remaining carbocation.
Size exclusion chromatography (SEC) is a chromatography technique that separates components in a mixture based on size. SEC columns use a porous stationary phase; therefore, smaller particles travel through the column and get stuck in the pores of the stationary phase while larger particles move faster through it. A student runs an SEC column and obtains the following result:
The blue substance has a smaller molecular weight than the red substance.
For a solvent to be used to crystallize a compound, which of the following characteristics are ideal?
The compound should be insoluble in the solvent at room temperature but soluble at high temperatures.
Which of the statements BEST describes this visual of a mixture of diethyl ether and water?
The phase on top is the organic phase and contains the volatile organic compounds.
Why is the eluate collected in fractions?
The separation of compounds is preserved.
Which statement about recrystallization is correct?
The solubility of the product typically increases in solvents that are heated.
The arrow points to the top of the mobile phase, which is called ____.
The solvent front
What is the first step of manually loading and injecting a sample?
Turn the injector handle to 'Load'.
In an immiscible mixture of benzene and water, the total pressure of the system can be described as:
iv and v
Which of the following statements about the formula is NOT correct?
xA is the number of moles of substance A.
Silica gel is a form of silicon dioxide. Listed below are the different intermolecular forces. Which of these BEST describes the major contributors of interactions that can occur between a compound and silica gel? i. Dipole-dipole interactions ii. Covalent bonding iii. Hydrogen bonding iv. Ionic bonding
(i) and (iii)
Using the plot of dew point vs. distilled volume, what is the boiling point of the more volatile component, cyclohexane?
81°C
Using the boiling point diagram for cyclohexane and toluene, what is the composition of the vapor phase at 84 °C?
95% cyclohexane, 5% toluene
What is bumping?
A large pocket of solvent vapor rapidly forms, displacing the surrounding liquid.
What can you use to predict the number of theoretical plates you need for your distillation?
Boiling point composition curve
A chemist separates three unknown compounds with TLC using a silica-gel stationary phase. Which option lists these compounds from the most polar to the least?
C>A>B
Which type of solvent movement is essential for a TLC analysis?
Capillary movement
Which of the following does NOT decrease the rate of SN2 reactions?
Decreased bond strength between the leaving group and the α-carbon.
How does steam distillation differ from fractional distillation?
Fractional distillation is used for miscible liquids, whereas steam distillation can be used for immiscible liquids.
A compound has a retardation factor of 0.5 in a TLC setup when run with a 30:70 acetone:hexane mixture. What would happen to the retardation factor if the solvent is changed to 50:50 acetone:hexane mixture?
It will be larger than 0.5.
Which of the following statements about TLC is correct?
More polar solutes will have greater attraction to the stationary phase and travel less.
A biochemist receives a patient sample that is thought to contain a prion, a pathogenic protein product caused by the misfolding of a host/native protein. The hydrophobicity, charge, and polarity are similar to the host proteins, but the misfolded protein has a different structure than the native protein. Which of the following chromatography techniques is the BEST method to isolate the prion?
Size exclusion chromatography
What occurs in an SN2 reaction?
Slow nucleophilic back-attack on the α-carbon, then fast departure of the leaving group.
This is an example of:
Solid-liquid Extraction
Liquid-liquid extractions are based on which of the following properties of a solute?
Solubility
Two undergraduate students are attempting to extract the essential oils found in mint leaves. Each student correctly assembles a steam distillation apparatus to extract the essential oils. Student A allows the water rate to be replenished at the same rate as the distillation. Student B attempts to speed up the rate of distillation by increasing the temperature and decreasing the rate of water replenishment. Which of the following statements BEST describes the likely experimental outcome?
Student A may have a higher yield of the correct volatile organic oils than Student B.
A student performed an experiment to separate the components of green ink from a pen. From the pen specifications, the student knows that the blue dye is more hydrophobic than the yellow dye. The student performed three column chromatography separations. He performed one separation using ethanol as the mobile phase, one separation using cyclohexane as the mobile phase, and one separation using water as the mobile phase. The image above is a snapshot of the ink separating in a column, but the student forgot to label the column. Based on the given information, which of the following statements is most likely CORRECT? The column utilized ethanol as the mobile phase.
The column utilized cyclohexane as the mobile phase.
Following steam distillation and extraction, the organic phase containing the essential oils of interest is treated with magnesium sulfate. Why is this treatment necessary?
The magnesium sulfate takes up excess water to become a hydrated compound, removing water contamination.
Which of these is NOT true of a good recrystallization solvent?
The solvent has many impurities.
Which of the following statements about the solvent used in column chromatography is CORRECT?
The solvent is known as the mobile phase.
A student ran a TLC experiment with three different compounds. None of the compounds moved far enough to determine Rf values. Which of the following is the MOST LIKELY cause of immobility of the compounds?
The solvent wasn't polar enough.
A student extracted cellulose, caffeine, and benzoic acid from a mixture of the three. They recovered a large amount of cellulose but very little caffeine and benzoic acid. What is the MOST LIKELY experimental cause?
The student did not properly dissolve the starting material in the dichloromethane.
A fellow undergraduate student is performing an acid-base extraction with a solution of dichloromethane containing caffeine and benzoic acid. Confident that they remember the next step, the student adds 3 M sodium hydroxide to their organic mixture. Which of the following is the MOST LIKELY experimental outcome?
The student will recover mostly caffeine and very little benzoic acid.
A student mixes their aqueous reaction mixture with acetone to perform a liquid-liquid extraction. After shaking the mixture in a separatory funnel and allowing the mixture to settle, which of the following is the expected experimental outcome?
There are no phases present, only one homogenous solution.
A student set up a vacuum filtration system by placing a Büchner funnel directly over a vacuum flask with a sidearm. He then connected the sidearm to the vacuum via vacuum tubing. Next, he placed a filter paper inside the funnel, turned the vacuum on and wetted the filter paper. When he finally poured the solution to be filtered, the filtrate dripped very slowly and then stopped. When he looked around, he saw that the filtrates in other students' setup quickly collected at the bottom. What is the most likely explanation for the failure of his setup?
There was no vacuum seal between the Buchner funnel and the flask
An entrepreneur designed a 'build-your-own-crystals' kit to be used in elementary school classrooms. When she demonstrated her product to a focus group of educators, she was asked what would happen to the crystals if they cooled the solvent slowly instead of using an ice bath. Which answer is correct?
There will be fewer crystals but they will be larger.
When should you NOT use simple distillation?
Two liquids with boiling points less than 30 °C apart.
You are tasked with separating a mixture containing multiple dyes, each with different polarities and hydrophobicity. You first separate fractions using their hydrophobicity, identifying two fractions. You have reason to believe that these fractions still contain 2 or more dyes in them. You prepare a silica gel column and run the first fraction through, using water as the solvent. This fails to further separate the fraction. What should your next step be?
You should try using two solvents with different polarity.
A Grignard reaction is the reaction between a Grignard reagent and...
a carbonyl-containing compound.
To improve separation on the plate, the spot should be...
as small as possib;e
A theoretical plate is...
equivalent to one vaporization-condensation cycle, or one simple distillation.
If the impurities are less soluble in the recrystallization solvent than the solute...
the impurities can be removed by hot filtration before proceeding with recrystallization.
