CHEM 2364 Midterm Test Questions
Fully Solved.
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)
Fully Solved.
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)
