Chem 43A Final
Exams Questions
and Complete
Solution
Denning [Date] [Course title]
,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. - Correct Answer Step 1: A portion of the solvent (in this case benzene) should
be heated.
Step 2: The minimum amount of hot solvent should be added to the sample to dissolve it.
Step 3: The hot solution should be allowed to cool to room temperature and, once no further
crystallization is taking place at room temperature, cooled in an ice-water bath.
Step 4: The crystals need to be isolated, for example, by centrifugation.
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? - Correct Answer Biphenyl is highly soluble in both hot and cold
benzene. Therefore, a large proportion of the biphenyl remains dissolved in the solvent when the
solution is cooled.
Solvents for crystallization must be carefully selected so that the compound being purified is soluble in
the hot solvent but insoluble in the cold solvent.
Water was found to be a good solvent for the recrystallization of organic substance X.
1. Which line correctly shows the solubility vs. temperature profile for this substance?
2. Suppose 0.1 g of X and 1.0 mL of water were mixed and heated to 80 °C. Would all of substance X
dissolve?
3. The solution prepared in (b) is cooled. At what temperature will crystals of X appear?
4. Suppose the cooling described in (c) were continued to 0 °C. How many grams of X would come out of
solution? - Correct Answer 1. The correct line is C. Water is a good solvent choice for the recrystallization
of X because X has a low solubility in water at low temperatures, but a high solubility in water at high
temperatures.
2. All of the substance X would dissolve at 80 °C. A solubility of 17.0 g in 100 mL of water is equivalent to
a solubility of 0.17 g in 1 mL of water. Since there is only 0.1 g of X, this is a greater solubility than is
required.
,3. Crystals will start to form when the solubility is reduced to 0.1 g/mL (equivalent to 10 g/100 mL on the
graph). Therefore, crystals of X should appear around 56 °C.
4. The recovery of X would amount to 0.085 g. A solubility of 1.5 g in 100 mL of water at 0 °C is
equivalent to a solubility of 0.015 g in 1 mL of water. Therefore, 0.015 g of X would remain dissolved in
the water, with the remainder being formed as crystals.
Which of the following statements is true when considering temperature during a recrystallization?
More than one answer may be correct. - Correct Answer Ideally the hot solvent used will keep the
impurities dissolved OR not dissolve them at all.
It is important that the sample is dissolved in just enough hot solvent.
If too much solvent is added, it may not be possible to retrieve the entire sample as it will have a degree
of solubility in the cold solvent. If the impurities remain in solution, they can be washed away. If they
remain undissolved, they can be filtered off before recrystallization.
Gravity Filtration - Correct Answer 1. removed powdered decolorizing charcoal from 20 ml of solution
(Gravity filtration using a fluted filter)
-Dissolved materials cannot be removed by filtration; however, the impurities can be removed on a silica
gel or alumina column. Decolorizing carbon (pelletized Norit) may also be used.
2. Remove solid impurities from 5 mL of liquid at room temperature.
(Gravity filtration using a filtering pipet)
Fluted Filters - Correct Answer The volume of liquid to be filtered is greater than about 10 mL, and solid
impurities are removed from a solution; often used in crystallization procedures.
1. removed powdered decolorizing charcoal from 20 ml of solution
(Gravity filtration using a fluted filter)
, -Dissolved materials cannot be removed by filtration; however, the impurities can be removed on a silica
gel or alumina column. Decolorizing carbon (pelletized Norit) may also be used.
Filtering Pipets - Correct Answer Used with volumes less than about 10 mL to remove solid impurities
from a liquid.
1. Remove solid impurities from 5 mL of liquid at room temperature.
(Gravity filtration using a filtering pipet)
Vacuum filtration - Correct Answer 1. Isolate 2.0 g of crystals from about 50 mL of solution after
performing a crystallization.
(Vacuum filtration with a Hirsch or Büchner funnel)
Buchner funnels - Correct Answer Primarily used to collect a desired solid from a liquid when the volume
is greater than about 10 mL; used frequently to collect the crystals obtained from crystallization.
1. Isolate 2.0 g of crystals from about 50 mL of solution after performing a crystallization.
(Vacuum filtration with a Hirsch or Büchner funnel)
Hirsch Funnels - Correct Answer Used in the same way as Büchner funnels, except the volume of liquid is
usually smaller (1−10 mL).
1. Isolate 2.0 g of crystals from about 10 mL of solution after performing a crystallization.
(Vacuum filtration with a Hirsch or Büchner funnel)
Filter-tip pipets - Correct Answer May be used to remove a small amount of solid impurities from a small
volume (1−2 mL) of liquid; also useful for pipetting volatile liquids, especially in extraction procedures.
1. Remove a very small amount of dirt from 1 mL of liquid.
Craig tubes - Correct Answer Used to collect a small amount of crystals resulting from crystallizations in
which the volume of the solution is less than 2 mL.
1. Collect crystals obtained from crystallizing a substance from about 1 mL of solution.
Exams Questions
and Complete
Solution
Denning [Date] [Course title]
,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. - Correct Answer Step 1: A portion of the solvent (in this case benzene) should
be heated.
Step 2: The minimum amount of hot solvent should be added to the sample to dissolve it.
Step 3: The hot solution should be allowed to cool to room temperature and, once no further
crystallization is taking place at room temperature, cooled in an ice-water bath.
Step 4: The crystals need to be isolated, for example, by centrifugation.
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? - Correct Answer Biphenyl is highly soluble in both hot and cold
benzene. Therefore, a large proportion of the biphenyl remains dissolved in the solvent when the
solution is cooled.
Solvents for crystallization must be carefully selected so that the compound being purified is soluble in
the hot solvent but insoluble in the cold solvent.
Water was found to be a good solvent for the recrystallization of organic substance X.
1. Which line correctly shows the solubility vs. temperature profile for this substance?
2. Suppose 0.1 g of X and 1.0 mL of water were mixed and heated to 80 °C. Would all of substance X
dissolve?
3. The solution prepared in (b) is cooled. At what temperature will crystals of X appear?
4. Suppose the cooling described in (c) were continued to 0 °C. How many grams of X would come out of
solution? - Correct Answer 1. The correct line is C. Water is a good solvent choice for the recrystallization
of X because X has a low solubility in water at low temperatures, but a high solubility in water at high
temperatures.
2. All of the substance X would dissolve at 80 °C. A solubility of 17.0 g in 100 mL of water is equivalent to
a solubility of 0.17 g in 1 mL of water. Since there is only 0.1 g of X, this is a greater solubility than is
required.
,3. Crystals will start to form when the solubility is reduced to 0.1 g/mL (equivalent to 10 g/100 mL on the
graph). Therefore, crystals of X should appear around 56 °C.
4. The recovery of X would amount to 0.085 g. A solubility of 1.5 g in 100 mL of water at 0 °C is
equivalent to a solubility of 0.015 g in 1 mL of water. Therefore, 0.015 g of X would remain dissolved in
the water, with the remainder being formed as crystals.
Which of the following statements is true when considering temperature during a recrystallization?
More than one answer may be correct. - Correct Answer Ideally the hot solvent used will keep the
impurities dissolved OR not dissolve them at all.
It is important that the sample is dissolved in just enough hot solvent.
If too much solvent is added, it may not be possible to retrieve the entire sample as it will have a degree
of solubility in the cold solvent. If the impurities remain in solution, they can be washed away. If they
remain undissolved, they can be filtered off before recrystallization.
Gravity Filtration - Correct Answer 1. removed powdered decolorizing charcoal from 20 ml of solution
(Gravity filtration using a fluted filter)
-Dissolved materials cannot be removed by filtration; however, the impurities can be removed on a silica
gel or alumina column. Decolorizing carbon (pelletized Norit) may also be used.
2. Remove solid impurities from 5 mL of liquid at room temperature.
(Gravity filtration using a filtering pipet)
Fluted Filters - Correct Answer The volume of liquid to be filtered is greater than about 10 mL, and solid
impurities are removed from a solution; often used in crystallization procedures.
1. removed powdered decolorizing charcoal from 20 ml of solution
(Gravity filtration using a fluted filter)
, -Dissolved materials cannot be removed by filtration; however, the impurities can be removed on a silica
gel or alumina column. Decolorizing carbon (pelletized Norit) may also be used.
Filtering Pipets - Correct Answer Used with volumes less than about 10 mL to remove solid impurities
from a liquid.
1. Remove solid impurities from 5 mL of liquid at room temperature.
(Gravity filtration using a filtering pipet)
Vacuum filtration - Correct Answer 1. Isolate 2.0 g of crystals from about 50 mL of solution after
performing a crystallization.
(Vacuum filtration with a Hirsch or Büchner funnel)
Buchner funnels - Correct Answer Primarily used to collect a desired solid from a liquid when the volume
is greater than about 10 mL; used frequently to collect the crystals obtained from crystallization.
1. Isolate 2.0 g of crystals from about 50 mL of solution after performing a crystallization.
(Vacuum filtration with a Hirsch or Büchner funnel)
Hirsch Funnels - Correct Answer Used in the same way as Büchner funnels, except the volume of liquid is
usually smaller (1−10 mL).
1. Isolate 2.0 g of crystals from about 10 mL of solution after performing a crystallization.
(Vacuum filtration with a Hirsch or Büchner funnel)
Filter-tip pipets - Correct Answer May be used to remove a small amount of solid impurities from a small
volume (1−2 mL) of liquid; also useful for pipetting volatile liquids, especially in extraction procedures.
1. Remove a very small amount of dirt from 1 mL of liquid.
Craig tubes - Correct Answer Used to collect a small amount of crystals resulting from crystallizations in
which the volume of the solution is less than 2 mL.
1. Collect crystals obtained from crystallizing a substance from about 1 mL of solution.
