Chem 43A - Exam 1

Safety Exam Questions + Other Safety Types

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

Pure substances melt over a ___ temperature range

narrow

Mixtures melt at a ___ and ___ temperature range than pure components

lower; broader

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

hot

How fast should you heat the melting point sample?

Heat quickly until you get close to the expected temperature

What is the effect of impurities in the sample?

The melting point will be depressed

When adding 127.3 and 1.578, the result on the calculator is 128.878. Round the answer to the appropriate precision based on the precision of the numbers in the sum.

128.9

A student is required to perform a crystallization on an impure sample of biphenyl. The sample weighs 0.5 g, 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 minimum amount of hot benzene to dissolve solvent

3) Cool solution, first to room temperature, then in an ice water bath

4) Isolate the biphenyl crystals by centrifugation

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

• ideally the hot solvent used will keep the impurities OR not dissolve them at all

• it is important that the sample is dissolved in just enough solvent

Fluted Filters

The volume of the liquid filtered is greater than about 10 mL, and solid impurities are removed from the solution

Filtering Pipets

Used with volumes less than about 10 mL to remove solid impurities from a liquid

Buchner Funnels

Primarily used to collect a desired solid when the volume is greater than about 10 mL

Hirsch Funnels

Used in the same way as Buchner Funnels but for volumes less than 10 mL

Filter-Tip Pipet

May be used to remove a small amount of solid impurities from a small volume (1−2 mL) of liquid

Craig Tubes

Used to collect a small amount of crystals resulting from crystallizations in which the volume of the solution is less than 2 mL

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

No crystals will be produced

Which of the following would be expected to be the most soluble in water? pentane vs ethyl methyl ketone

ethyl methyl ketone (is the polar compound)

SAFETY: 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 to let you know it is hot.

False

SAFETY: When performing a filtration under vacuum, always make sure that:

the collection flask is securely clamped

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

• You should ensure that no one else is using an open flame near your experiment.

• You should not use an open flame to heat the solvent.

In a solvent extraction using a separatory funnel, the densities of the two liquids determine whether the organic solvent is the top layer or the bottom layer.
Using the table of densities, determine whether chloroform is the top layer or the bottom layer when used with water. (density of chloroform = 1.50 g/cm³, density of water = 1.00 g/cm³)

bottom layer

SAFETY: What is the purpose of venting the separatory funnel?

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

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

The powder no longer forms clumps.

Solvents: Which of the following are hazards associated with dichloromethane?

• harmful by inhalation

• It can dissolve some of the fatty tissues in the skin with prolonged contact, causing chemical burns

• limited evidence of a carcinogenic effect

SAFETY: Which of the following are advantages of performing experiments at microscale level? More than one answer may be correct.

• A reduced risk of fire or explosion in the laboratory

• Reduced production of hazardous and polluting waste

• Reduced contact with hazardous materials that may be harmful to health

Microscale experiments use small quantities of chemicals, reducing risks involved in handling and heating chemicals, but safety precautions should still be used

Sublimation can be used to purify your product if:

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

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 - the lower the external pressure, the more readily the solid sublimes and thus less heat needs to be supplied

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. - the vapor pressure of the solid is much lower than the atmospheric pressure at the melting point, so the solid will melt rather than sublime

Which of the following would be expected to form hydrogen bonds with water? (hexane, acetaldehyde, methyl alcohol, N-ethylacetamide)

acetaldehyde, methyl alcohol, N-ethylacetamide

hydrogen bonding occurs when the donor and the acceptor both have one of the highly electronegative atoms: O, N, or F. If all the H are attached to C, then hydrogen bonding will not occur

For each of the following pairs of solutes and solvent, predict whether the solute would be soluble or insoluble.

• malic acid in water

• naphthalene in water

• amphetamine in ethyl alcohol

• aspirin in water

• succinic in hexane

• Ibuprofen in diethyl ether

• 1-Decanol (n-decyl alcohol) in water.

• Malic acid, soluble in water

• Naphthalene, insoluble in water

• Amphetamine, soluble in alcohol

• Aspirin, slightly soluble in water; essentially insoluble

• Succinic acid, insoluble in hexane

• Ibuprofen, soluble in diethyl ether

• 1-Decanol, insoluble in water

Predict whether the following pairs of liquids would be miscible or immiscible:

• hexane and benzene

• methylene chloride and benzene

• water and toluene

• ethyl alcohol and isopropyl alchol

• water and methyl alcohol

• Water and methyl alcohol, miscible

• Hexane and benzene, miscible

• Methylene chloride and benzene, miscible

• Water and toluene, immiscible

• Ethyl alcohol and isopropyl alcohol, miscible

Saturated aqueous sodium chloride (d = 1.2 g/mL) is added to the following mixtures in order to dry the organic layer. Which layer is likely to be on the bottom in each case?

1. Sodium chloride layer or a layer containing a high-density organic compound dissolved in methylene chloride (d = 1.4 g/mL)?

2. Sodium chloride layer or a layer containing a low-density organic compound dissolved in methylene chloride (d = 1.1 g/mL)?

1. high-density organic compound dissolved in methylene chloride

2. sodium chloride

Denser liquids will be on the bottom

Correlate the procedures needed to accomplish the given steps (1-6) to separate a mixture containing the following compounds.

1. All three compounds are in the organic solution.

2. The salt (C6H4)NO2COO-Na+ partitions to the aqueous layer and can be removed.

3. The salt (C6H4)BrNH3+Cl- partitions to the aqueous layer and can be removed.

4. The majority of the water in the diethyl ether moves into the aqueous layer and can be separated.

5. The small amount of water remaining in the diethyl ether is taken up by the solid and can be filtered off.

6. Benzoin is obtained.

1. Dissolve the mixture in diethyl ether.

2. Add aqueous NaOH.

3. Add aqueous HCl.

4. Add saturated aqueous NaCl solution. - brine

5. Dry organic layer over anhydrous sodium sulfate.

6. Evaporate off the diethyl ether.

When using a separatory funnel, which layer should be discarded?

It depends on the conditions - depending on which solvents are used

SAFETY: When venting a separatory funnel, which of the following statements is correct?

Where possible, vent the separatory funnel in a fume hood.