Experiment 1/2: Identification and Purification of Unknown Compounds via Macroscale and Microscale Recrystallization + Microscale BP Determination

Question 1

What physical property will help you to determine the identity of your unknown acid?

Answer 1

melting point

Question 2

What does it mean if a mixed solvent system such as ethanol/water is suggested for a recrystallization?

Answer 2

Heat the sample in a minimal amount of hot ethanol and add drops of hot or cold water until the cloud point is reached, then allow the solution to cool.

Question 3

How will the melting point change if the solid sample is impure?

Answer 3

broader than expected

Question 4

What are the properties of a good recrystallization solvent?

Answer 4

A good solvent dissolves the solid of interest when the solvent is hot, but not when the solvent is cold or at room temperature.

Question 5

If you have a dissolved colored impurity (and some solid contaminants) when you dissolve your unknown in the appropriate solvent, what should be done?

Answer 5

Add activated charcoal to the solution of unknown in your solvent before you do the gravity filtration.

Question 6

! Why is filter paper fluted for a gravity filtration?

Answer 6

It allows more surface area of the paper to come in contact with the mixture being filtered.

Question 7

When you collect your crystals at the end of lab, why do you wash with cold solvent (as opposed to room temp or hot solvent)?

Answer 7

Cold solvent will be much less likely to re-dissolve the solid during the wash.

Question 8

If 0.67 g of pure solid is collected from 0.85 g of crude solid, what is the percent recovery?

Answer 8

79%

Question 9

! When using a mixed solvent system such as water and methanol to do a recrystallization, which of the following is TRUE?

Answer 9

The first step in the recrystallization process is to dissolve the compound in a minimum of one of the hot solvents.

Question 10

Which technique should you used to collect your purified crystals?

Answer 10

Vacuum filtration

Question 11

What is a sign that a solid sample may contain an impurity?

Answer 11

The solid has a broad melting point range (>2 degrees C span) and a value that is lower than the literature value.

Question 12

What does it mean if a mixed solvent system such as ethanol/water is suggested for a recrystallization?

Answer 12

Heat the sample in a minimal amount of hot ethanol and add drops of hot or cold water until the cloud point is reached, then allow the solution to cool.

Question 13

What are the properties of a good recrystallization solvent?

Answer 13

A good solvent dissolves the solid of interest when the solvent is hot, but not when the solvent is cold or at room temperature.

Question 14

! When you collect your crystals at the end of lab, why do you wash with cold solvent (as opposed to room temp or hot solvent)?

Answer 14

Cold solvent will be much less likely to re-dissolve the solid during the wash.

Question 15

What is the best way to remove an insoluble solid, like sand, from an impure solid sample for recrystallization?

Answer 15

Find a solvent to dissolve the desired compound and then do a hot gravity filtration.

Question 16

Why do organic chemists often rinse glassware with an organic solvent, like acetone or an alcohol, after washing with soap and water?

Answer 16

Sometimes the compounds that we work with in an organic lab are not highly soluble in water, so even after scrubbing glassware with soap and water, the glassware may contain a residue. Using an organic solvent (like acetone) will rinse our any remaining nonpolar/slightly polar residues.

Question 17

In which waste container should dichloromethane (DCM, CH2Cl2) be disposed?

Answer 17

halogenated waste container

Question 18

What physical property will help you to determine the identity of your unknown solid?

Answer 18

melting point

Question 19

In which waste container should the liquid solvent, acetone (C3H6O) be disposed?

Answer 19

non-halogenated waste container

Question 20

Solvents in crystallization should be?

Answer 20

miscible

Question 21

After crystals are collected, they should be washed with?

Answer 21

cold solvent

Question 22

For crystallization, the solution should be?

Answer 22

saturated

Question 23

An impure compound was dissolved in hot methanol and a gravity filtration was done to remove activated charcoal that was added to the solution. During the filtration, crystals began to form on the filter paper. What should have been done next?

Answer 23

Rinse the filter paper with hot methanol.

Question 24

Which of the following will result in the formation of few or no crystals when trying to recrystallize a solid?

Answer 24

Using an excessive amount of solvent to dissolve the impure solid.

Question 25

An agent used for decolorization?

Answer 25

activated charcoal

Question 26

Dichloromethane (DCM) is used as a recrystallization solvent. The solid is collected by vacuum filtration. Where should the filtrate be disposed?

Answer 26

Placed in the halogenated waste container.

Question 27

How are insoluble impurities from the solution removed during crystallization?

Answer 27

filtration

Question 28

When using Siwoloboff’s method, when is the thermometer read to determine the boiling point of the unknown liquid?

Answer 28

When the entire setup is collected down and the liquid unknown begins to enter the capillary tube.

Question 29

Which of the following is a good practice for obtaining an accurate melting point of a solid?

Answer 29

Allowing the temperature to increase at a rate of 2 degrees C once the temperature reaches 20 degrees C below the expected melting point

Question 30

After the crystals are collected, they should be washed with?

Answer 30

cold solvent

Question 31

A microscale boiling point determination has been correctly set up and is heating. At the start, a small stream of air bubbles exits from the capillary tube. Then the rate of bubbling becomes faster and faster. As the rate of bubbling becomes faster, what is causing the bubbling?

Answer 31

The bubbles are actually the vapor of the unknown liquid being forced out of the tube.

Question 32

What does it mean if a mixed solvent system such as ethanol/water is suggested for a recrystallization?

Answer 32

Heat in a minimal amount of hot ethanol and add drops of water until the cloud point is reached the allowed to cool.

Question 33

When using Siwoloboff’s method, when is the thermometer read to determine the boiling point of the unknown liquid?

Answer 33

When the entire setup is collected down and the liquid unknown begins to enter the capillary tube

Question 34

When you set up for microscale boiling point determination, you are asked to rubber band a thermometer to the test tube containing the unknown liquid. Where should you positive the bulb of the thermometer?

Answer 34

It should be aligned with the curved bottom of the best tube containing your unknown.

Question 35

When a crystallized product is collected via vacuum filtration,

Answer 35

the crystals should be rinsed with small aliquots of cold solvent.

Question 36

When you set up for microscale boiling point determination, you are asked to put a capillary tube into the test tube containing your unknown liquid. How should the capillary tube be inserted?

Answer 36

open end down in the liquid

Question 37

Solvents in crystallization should be?

Answer 37

miscible

Question 38

How are insoluble impurities from solution during crystallization removed?

Answer 38

filtration