CHEM 2364 Midterm Exam Questions
With Verified Answers
Criteria for Selecting the Solvent for Recrystallization - ANSWER 1. The
compound of interest should be insoluble at cold temperatures or soluble at high
temperatures.
2. The impurities should be insoluble or soluble at all temperatures
3. The solvent should be able to be removed from solid therefore having a low
boiling point.
4. Safe for environment
Solubility and Temperature - ANSWER Solubility increases with increasing
temperature
Solubility and Polarity - ANSWER "Like dissolves Like" so nonpolar dissolves in
nonpolar solvents etc.
Solubility and IMF - ANSWER The stronger the IMF, the greater the solubility of
the solute in the solvent.
Melting Point and Purity - ANSWER Impurities generally lower the melting point
due to interrupting IMF in perfect crystalline form and requires less energy to
break bonds. HOWEVER, if impurities are in a large quantity then it will increase
melting point.
Percent Recovery - ANSWER mass of recrystallized aspirin / mass of aspirin in
original sample x 100%
Recrystallization - ANSWER a type of separation, the separation of the
substance of interest from impurities, which does include any solvents
Steps of Filtration - ANSWER Part 2 will allow for separation between aspirin and
impurities on basis of solubility in water. With water having a high boiling point, it
cannot be easily removed from recrystallized solid thus the third part of this
experiment is focused on removing water.
Part 3 focuses on using ethanol (lower boiling point and better at drying)- water
mixture to aid in the removal of the solvent without much loss from dissolution.
Aspirin solubility in water - ANSWER only slightly
Distillation - ANSWER a mixture of two liquid can be separated; two different
liquids are different boiling points molecule A with lower BP will evaporate into
, gas phase before the other molecule B. When it evaporates into gas phase, it
can be condensed into a purified liquid called the distillate in a separate
container.
IMF and Boiling Point - ANSWER The stronger the IMF, the higher the BP
Simple Distillation - ANSWER separate substances with wide far apart boiling
points (75 degrees Celsius); some molecules from liquid B will evaporate below
BP and some of liquid A will not evaporate. Likely Liquid A with lower BP will
remain in mixture with molecule B
Fractional Distillation - ANSWER improves separation since it allows for many
small distillation steps; uses Vigreux column where gas can condense and re-
evaporate on the way up before reaching the final condensing step
Good Fractional Distillation - ANSWER plateau for a while around the boiling
point of each liquid
Poor Fractional Distillation - ANSWER increase in temperature at the same rate
for the entire length of the distillation
Sand bath - ANSWER 1/3 to 1/2 full; poor heat source but will heat evenly and
can be adjusted
Joint Grease - ANSWER the purpose is to not have the glass pieces break when
pulling apart after experiment
Condenser - ANSWER Water goes in at the bottom (right) bard and out of the top
(left) barb; goal is to cool and condense vapors
Vigreux Column - ANSWER gas can condense and reevaporate on the way up
before reaching the final condensing step
Thermometer Placement in Distillation Set-Up - ANSWER The probe needs to be
placed slightly below the side arm of the three way adapter (little higher). It will
read the temperature of the vapor just as it condenses
Distilling to Dryness - ANSWER Risk of explosion
Why density is affected by temperature? - ANSWER Heating a substance causes
molecules to speed up and spread slightly further apart, occupying a larger
volume that results in a decrease in density. (density decreases as temperature
increases)
Density - ANSWER the measure of how heavy something is compared to its size
With Verified Answers
Criteria for Selecting the Solvent for Recrystallization - ANSWER 1. The
compound of interest should be insoluble at cold temperatures or soluble at high
temperatures.
2. The impurities should be insoluble or soluble at all temperatures
3. The solvent should be able to be removed from solid therefore having a low
boiling point.
4. Safe for environment
Solubility and Temperature - ANSWER Solubility increases with increasing
temperature
Solubility and Polarity - ANSWER "Like dissolves Like" so nonpolar dissolves in
nonpolar solvents etc.
Solubility and IMF - ANSWER The stronger the IMF, the greater the solubility of
the solute in the solvent.
Melting Point and Purity - ANSWER Impurities generally lower the melting point
due to interrupting IMF in perfect crystalline form and requires less energy to
break bonds. HOWEVER, if impurities are in a large quantity then it will increase
melting point.
Percent Recovery - ANSWER mass of recrystallized aspirin / mass of aspirin in
original sample x 100%
Recrystallization - ANSWER a type of separation, the separation of the
substance of interest from impurities, which does include any solvents
Steps of Filtration - ANSWER Part 2 will allow for separation between aspirin and
impurities on basis of solubility in water. With water having a high boiling point, it
cannot be easily removed from recrystallized solid thus the third part of this
experiment is focused on removing water.
Part 3 focuses on using ethanol (lower boiling point and better at drying)- water
mixture to aid in the removal of the solvent without much loss from dissolution.
Aspirin solubility in water - ANSWER only slightly
Distillation - ANSWER a mixture of two liquid can be separated; two different
liquids are different boiling points molecule A with lower BP will evaporate into
, gas phase before the other molecule B. When it evaporates into gas phase, it
can be condensed into a purified liquid called the distillate in a separate
container.
IMF and Boiling Point - ANSWER The stronger the IMF, the higher the BP
Simple Distillation - ANSWER separate substances with wide far apart boiling
points (75 degrees Celsius); some molecules from liquid B will evaporate below
BP and some of liquid A will not evaporate. Likely Liquid A with lower BP will
remain in mixture with molecule B
Fractional Distillation - ANSWER improves separation since it allows for many
small distillation steps; uses Vigreux column where gas can condense and re-
evaporate on the way up before reaching the final condensing step
Good Fractional Distillation - ANSWER plateau for a while around the boiling
point of each liquid
Poor Fractional Distillation - ANSWER increase in temperature at the same rate
for the entire length of the distillation
Sand bath - ANSWER 1/3 to 1/2 full; poor heat source but will heat evenly and
can be adjusted
Joint Grease - ANSWER the purpose is to not have the glass pieces break when
pulling apart after experiment
Condenser - ANSWER Water goes in at the bottom (right) bard and out of the top
(left) barb; goal is to cool and condense vapors
Vigreux Column - ANSWER gas can condense and reevaporate on the way up
before reaching the final condensing step
Thermometer Placement in Distillation Set-Up - ANSWER The probe needs to be
placed slightly below the side arm of the three way adapter (little higher). It will
read the temperature of the vapor just as it condenses
Distilling to Dryness - ANSWER Risk of explosion
Why density is affected by temperature? - ANSWER Heating a substance causes
molecules to speed up and spread slightly further apart, occupying a larger
volume that results in a decrease in density. (density decreases as temperature
increases)
Density - ANSWER the measure of how heavy something is compared to its size
