CHEM 43A Pre-Lab Quizzes

After recrystallization, the final weight of biphenyl is found to be 0.02 g. Assume that all steps were carried out correctly, no spillages occurred, and the student lost minimal solid on the glassware or in transfers. Why is the recovery so low?

- Biphenyl is highly soluble in both hot and cold benzene - a large portion of the biphenyl remains dissolved in the cool solvent - you want the solute to be soluble in hot solvent and insoluble in cool solvent

Which of the following statements is true when considering temperature during recrystallization?

- It is important that the sample is dissolved in just enough hot solvent --> too much solvent = not retrievable - ideally the hot solvent used will keep the impurities dissolved or not dissolve them at all --> impurities can be washed away or filtered off

Which of the following solvents would be the best to separate a mixture containing benzoic acid, 2,4-dinitrobenzoic acid, and 2,4,6-trinitrobenzoic acid by TLC?

- acetone because the mixture contains many polar solvents

What functional group can give a false positive for Jones Test?

- aldehyde

Observing the formation of a silver mirror on the surface of a test tube when using Tollen's reagent indicates the presence of:

- an aldehyde

Which key reagent would you need to use to distinguish between a primary and secondary alcohol?

- anhydrous zinc chloride and hydrochloric acid

If you separated a mixture of benzene, toluene, and m-xylene by gas chromatography, what would be the expected order of retention times?

- benzene --> toluene --> m-Xylene

When separating a mixture containing, benzoic acid, 2,4-dinitrobenzoic acid, and 2,4,6-trinitrobenzoic acid, the ________ spot would be expected to travel the greatest distance up the plate as it is the _______ compound. The ________ would travel the least distance as it is the ______ component.

- benzoic acid - least polar - 2, 4, 6 trinitrobenzoic acid - most polar

Consider a sample that is a mixture composed of biphenyl, benzoic acid, and benzyl alcohol. Predict the order of elution of the components in this mixture. Assume that the chromatography uses a silica column and the solvent system is based on cyclohexane, with an increasing proportion of methylene chloride added as a function of time.

- biphenyl --> benzyl alcohol --> benzoic acid - cyclohexane is a nonpolar solvent so the most nonpolar compound will elute first

When performing a filtration under vacuum, always make sure that the ________ is securely clamped

- collection flask

What types of intermolecular forces are expected between CH3OH molecules?

- dipole forces - induced dipole forces - hydrogen bonding

While setting up a micro-boiling point determination you accidentally break a capillary tube. You should:

- dispose of it in the appropriate glass waste container

The more volatile a component the ________, hence the more it spends in the ________, giving it a ______ retention time. Therefore, components of a liquid mixture will elute in order of ______.

- greater its vapor pressure - gaseous mobile phase - shorter - increasing boiling point

How fast should you heat the melting point sample?

- heat quickly until you get close to the expected temperature

When using a melting point apparatus, take care not to burn yourself as the apparatus will get very ______

- hot

Predict the trend in boiling points for the following compounds (least --> greatest)

- iso-pentane --> pentane --> ethanol --> water --> sodium n-butoxide - strenght of intermolecular interactions: ionic > hydrogen bonding > dipole-dipole > london - boiling point increases with molecular weight and decreases with increasing branching

Which of the following hazards are associated with dichloromethane (methylene chloride)

- it can dissolve some of the fatty tissues in the skin with prolonged contact, causing chemical burns - harmful by inhalation - limited evidence of a carcinogenic effect

Sublimations are generally performed under reduced pressure because:

- less heat needs to be supplied for the vapor pressure of the solid to equal that of the external pressure - Sublimation occurs when the vapor pressure of a solid = external pressure; therefore, the lower the external pressure, the more readily a solid will sublime, and the less heat that needs to be supplied.

Pure substances melt over a _______ temperature range. Mixtures melt at a _________ and over a ________ temperature range

- narrow - lower temperature than the pure components - broader

What is the result of boiling the impure product with too much solvent and then cooling with ice?

- no crystals are produced

If the mixture of the unknown with aspirin shows _______ of melting point then the unknown is aspirin

- no depression or expansion

If the mixture of the unknown with urea shows ______ of melting point then the unknown is urea

- no depression or expansion

The stationary phase in TLC is ________. Therefore, the _______ component of the mixture will travel a greater distance up the plate, resulting in a _________ value for the component.

- polar - less polar - higher Rf

Gas chromatography separates compounds depending on their _________. Benzene, m-xyelene, and toluene have similar _____, therefore, the main basis for separation is _______.

- polarity and volatility - polarities - volatility

A risk assessment for a reaction should include the hazards associated with the:

- procedures involved - chemical products and by-products - chemical reagents used

Which of the following would be expected to be the most soluble in water?

- propanal - like dissolves like; water is a polar solvent and has hydrogen bonds - propanal is polar and capable of accepting a hydrogen bond from water

Which of the following are advantages of performing experiments at microscale level?

- reduced contact with hazardous materials that may be harmful to health - reduced production of hazardous and polluting waste - a reduced risk of fire or explosion in the laboratory

What reagent (or compound) causes the observed visual change in a positive Lucas test?

- the alkyl halide product is insoluble in water

What is the effect of impurities of the sample?

- the melting point will be depressed

What observation would you expect to make if a primary or secondary alcohol reacted with acidified potassium dichromate (VI) solution?

- the orange solution turns green

Why is it important to handle silica powder in a fume cupboard?

- the powder is fine and easily inhaled - the powder, if inhaled, can cause serious problems to the respiratory tract

When adding drying agent, how do you know when you have added enough?

- the powder no longer forms clumps

A solid substance has a vapor pressure of 100 mm Hg at the melting point (100 °C). Assuming an atmospheric pressure of 760 mm Hg, describe the behavior of this solid as the temperature is raised from room temperature to its melting point.

- the solid will show a normal solid to liquid transition at 100 C - solids will sublime if their vapor pressure are greater than atmospheric pressure - here, vapor pressure < atmospheric pressure so substances melts not sublimes

What happens if the spots are made too large when preparing a TLC plate for development?

- the spots will saturate the plate, show tailing, and/or diffuse into one another.

Where are you NOT supposed to be plastic keck clips?

- they are not supposed to be put anywhere they can melt - not too close to the heated part of the distillation apparatus

Why might you want to heat a reaction?

- to overcome the activation energy for the reaction - to increase the rate of the reaction (because of the greater kinetic energy of molecules)

What is the purpose of venting the separatory funnel?

- to relieve built up pressure from gases produced, or solvent vapor formed

Is ethyl acetate (density = 0.90 g/cm3) the top layer or the bottom layer when used with water?

- top layer

How long should you run a TLC plate for?

- until the solvent front is just below the top of the plate

An unknown compound melting at 134 C was suspected to be either aspirin (melting point = 135 C) or urea (melting point = 133 C) How could you determine whether one of these two suspected compounds was identical to the unknown compound without using any form of spectroscopy?

- using the mixed melting point procedure

When heating a flammable or volatile solvent for a recrystallization, which of these statements are correct?

- you should not use an open flame to heat the solvent - you should ensure no one else is using an open flame near your experiment

Sublimation can be used to purify your product if:

- your solid product has relatively high vapor pressure but the impurities have a low vapor pressure

A student is required to perform a crystallization on an impure sample of biphenyl. The sample weighs 0.5 grams and contains about 5% impurity. Based on their knowledge of solubility, they decide to use benzene as the solvent. Identify the steps that the student must perform, and place them in the correct order.

1. Heat a portion of the benzene solvent 2. Add the minimum amount of hot benzene to dissolve the sample 3. Cool solution, first to room temperature, then in an ice water bath 4. Isolate the biphenyl crystals by centrifugation

Name the steps to isolate pure isoppentyl acetate from the reaction mixture 1. this deprotonates unreacted acetic acid, making a water soluble salt. 2. this ensures that the evolution of carbon dioxide gas is complete 3. this removes byproducts 4. this removes water from the product 5. this separates the sodium sulfate from the ester

1. aqueous sodium bicarbonate is added to the reaction mixture 2. the mixture is stirred, capped, and gently shaken, with frequent venting 3. the lower aqueous layer is removed using a Pasteur pipette and discarded 4. the organic layer is dried over granular anhydrous sodium sulfate 5. the dry ester is decanted using a Pasteur pipette to a clean conical vial

Match each of the following situations with the most appropriate filtration device: 1. remove powdered decolorizing charcoal from 20 mL of solution 2. collect crystals obtained from crystallizing a substance from about 1 mL of solution 3. remove a very small amount of dirt from 1 mL of liquid 4. isolate 2.0 g of crystals from about 50 mL of solution after performing a crystallization. 5. remove dissolved colored impurities from about 3 mL of solution 6. remove solid impurities from 5 mL of liquid at room temperature

1. gravity filtration using a fluted filter 2. craig tube 3. filter tip pipet 4. vacuum filtration using a Hirsch or Buchner funnel 5. cannot be removed by filtration 6. gravity filtration using a filtering pipet

When adding 145.03 and 12.1, the result on the calculator is 157.13 Round the answer to the appropriate precision based on the precision of the numbers in the sum.

157.1

When using a hot plate or other heating device, you need to be aware of hot surfaces that could cause burns: a hot surface will always glow red or let you know it is hot.

False