CHEM 2364- midterm Exam and All
Correct Answers.
lab 1- rank the solvents by polarity (water & ethanol) - Answer water is MORE polar than
ethanol, this is bc water has two hydrogen bonds while alcohols have one
lab 1- what is the criteria for selecting a recrystallization solvent? - Answer -insoluble at cold
temperatures, but is soluble at higher temperatures.
-impurities should be insoluble in the solvent at all temps or soluble at all temps
-the solvent should be easily removable from the solid (aka: low boiling point)
-safe to use and minimizes environmental impact
lab 1-Understand how recrystallization works to purify a compound - Answer dissolving the
impure solid in a heated solvent until the solution reaches saturation, followed by gradual
cooling. As the solution cools, pure crystals form while impurities are left behind.
lab 1- what is the relationship between solubility and temperature? - Answer the solubility
increases at higher temperatures
lab 1- what is the relationship between solubility of aspirin and IMF's? - Answer the stronger
the IMF's, the greater the solubility
lab 1- what is the relationship between melting point and purity? - Answer pure substances
have a higher melting point and impurities lower the melting point
lab 1- what is the formula for percent recovery? - Answer % recovery = (mass of product) /
(mass of original sample) x 100
lab 1- what are the filtrates and precipitates in each step? - Answer step 1. filtrate is sucrose
and impurities and precipitate is aspirin & water
step 2. filtrate is water and precipitate is pure aspirin
lab 1- why was each reagent used in this order (water, then ethanol ) for recrystallization? -
Answer -water is used first bc it has the highest boiling point compared to the impurities
-ethanol is used to remove water
lab 2- how does IMF strength relate to boiling point? - Answer the stronger the IMF, the
higher the boiling point
, lab 2- Understand how distillation works to separate compounds. - Answer The liquid
mixture is heated until it boils.
The most volatile component vaporizes first.
The vapor passes through a condenser, where it cools and condenses back into a liquid.
The condensed liquid, called the distillate, is collected in a separate container.
lab 2- what is the difference between simple and fractional distillation? - Answer -simple
distillation is to separate substances in mixtures with widely differing boiling points
-fractional distillation is to separate substances in mixtures with similar boiling points
lab 2- Read a distillation graph (boiling point, liquid or gas percentage/mole fraction) - Answer
BP: look for plateaus or horizontal stretches
MF: vapor line on top
lab 2- why is each (sand bath, clamps, joint grease, condenser) important in the distillation set
up? - Answer -sand bath: heats the RBF more evenly than it just sitting on the hot plate, bc
its not just coming from below
-clamps: to hold the RBF and the vacuums connecting tube in place during distillation
-joint grease: to help separate the pieces after distillation bc they would get stuck if you didn't
use them
-condenser: water goes in at bottom, and out at the top bc this ensures that the condenser will
always be full of cool water
lab 2- where do you place the thermometer in the distillation set up, and why? - Answer the
tip of the thermometer gets placed at the juncture in the 3 way connector
-this is bc it allows it to record temperatures after the liquid boils, becomes a gas, and makes it
way to the column
-if it was too low it'd read the temperature before anything was condensed
-if it was placed at the end of the condenser, it would record the temp after it has cooled down
lab 2- why should you not distill to dryness? - Answer if the flask runs dry and the
temperature continues to rise, an explosion can occur or damage to the flask due to an
unintended chemical reaction
lab 2- why is density affected by temperature? - Answer heating a substance causes
molecules to speed up and spread further apart, which occupies a larger volume that results in
a decrease in density
-as heat increases, density decreases
Correct Answers.
lab 1- rank the solvents by polarity (water & ethanol) - Answer water is MORE polar than
ethanol, this is bc water has two hydrogen bonds while alcohols have one
lab 1- what is the criteria for selecting a recrystallization solvent? - Answer -insoluble at cold
temperatures, but is soluble at higher temperatures.
-impurities should be insoluble in the solvent at all temps or soluble at all temps
-the solvent should be easily removable from the solid (aka: low boiling point)
-safe to use and minimizes environmental impact
lab 1-Understand how recrystallization works to purify a compound - Answer dissolving the
impure solid in a heated solvent until the solution reaches saturation, followed by gradual
cooling. As the solution cools, pure crystals form while impurities are left behind.
lab 1- what is the relationship between solubility and temperature? - Answer the solubility
increases at higher temperatures
lab 1- what is the relationship between solubility of aspirin and IMF's? - Answer the stronger
the IMF's, the greater the solubility
lab 1- what is the relationship between melting point and purity? - Answer pure substances
have a higher melting point and impurities lower the melting point
lab 1- what is the formula for percent recovery? - Answer % recovery = (mass of product) /
(mass of original sample) x 100
lab 1- what are the filtrates and precipitates in each step? - Answer step 1. filtrate is sucrose
and impurities and precipitate is aspirin & water
step 2. filtrate is water and precipitate is pure aspirin
lab 1- why was each reagent used in this order (water, then ethanol ) for recrystallization? -
Answer -water is used first bc it has the highest boiling point compared to the impurities
-ethanol is used to remove water
lab 2- how does IMF strength relate to boiling point? - Answer the stronger the IMF, the
higher the boiling point
, lab 2- Understand how distillation works to separate compounds. - Answer The liquid
mixture is heated until it boils.
The most volatile component vaporizes first.
The vapor passes through a condenser, where it cools and condenses back into a liquid.
The condensed liquid, called the distillate, is collected in a separate container.
lab 2- what is the difference between simple and fractional distillation? - Answer -simple
distillation is to separate substances in mixtures with widely differing boiling points
-fractional distillation is to separate substances in mixtures with similar boiling points
lab 2- Read a distillation graph (boiling point, liquid or gas percentage/mole fraction) - Answer
BP: look for plateaus or horizontal stretches
MF: vapor line on top
lab 2- why is each (sand bath, clamps, joint grease, condenser) important in the distillation set
up? - Answer -sand bath: heats the RBF more evenly than it just sitting on the hot plate, bc
its not just coming from below
-clamps: to hold the RBF and the vacuums connecting tube in place during distillation
-joint grease: to help separate the pieces after distillation bc they would get stuck if you didn't
use them
-condenser: water goes in at bottom, and out at the top bc this ensures that the condenser will
always be full of cool water
lab 2- where do you place the thermometer in the distillation set up, and why? - Answer the
tip of the thermometer gets placed at the juncture in the 3 way connector
-this is bc it allows it to record temperatures after the liquid boils, becomes a gas, and makes it
way to the column
-if it was too low it'd read the temperature before anything was condensed
-if it was placed at the end of the condenser, it would record the temp after it has cooled down
lab 2- why should you not distill to dryness? - Answer if the flask runs dry and the
temperature continues to rise, an explosion can occur or damage to the flask due to an
unintended chemical reaction
lab 2- why is density affected by temperature? - Answer heating a substance causes
molecules to speed up and spread further apart, which occupies a larger volume that results in
a decrease in density
-as heat increases, density decreases
