Binghamton University CHEM 335 Organic
Chemistry Lab CA3 Answer Key | 2026
Update | 100% Correct.
SECTION 1: Laboratory Techniques & Safety (Questions 1–8)
Q1: During recrystallization of a crude organic solid, a student observes that the compound
dissolves completely in hot ethanol but does not crystallize upon cooling to room temperature.
Which step should be performed next to induce crystal formation?
A. Add excess ethanol and reheat to dissolve any remaining solid
B. Add a small amount of seed crystals to the cooled solution
C. Increase the cooling rate by placing the flask in an ice bath
D. Filter the solution while hot to remove insoluble impurities
Correct Answer: B
Rationale: Correct because seeding provides a nucleation surface for crystal growth, inducing
crystallization in a supersaturated solution that fails to crystallize spontaneously. Per organic
chemistry lab standard, seeding is the preferred first step before more aggressive cooling
methods.
Q2: A student is performing a simple distillation to purify a liquid with a boiling point of 156°C.
The thermometer is placed in the distilling flask rather than at the side arm of the still head. What
is the primary consequence of this error?
A. The distillate will be collected at a lower temperature than the true boiling point
B. The distillate will be collected at a higher temperature than the true boiling point
C. The thermometer will measure the temperature of the liquid, not the vapor, yielding inaccurate
boiling point data
D. The distillation will proceed faster due to better thermal contact
Correct Answer: C
Rationale: Correct because the thermometer must be positioned at the side arm to measure the
temperature of the vapor in equilibrium with the condensing liquid; placement in the flask
measures liquid temperature, which is not representative of the boiling point and yields
inaccurate data.
Q3: In a liquid-liquid extraction using a separatory funnel, a student is unsure which layer is the
organic phase. Which test should be performed to determine the identity of the layers?
A. Add a few drops of water to the funnel and observe which layer the drops join
B. Measure the density of each layer using a hydrometer
C. Add a small amount of sodium chloride to each layer and observe solubility
D. Remove a small sample of each layer and test with litmus paper
Correct Answer: A
, Rationale: Correct because adding water droplets to the separatory funnel and observing which
layer they join (miscible = aqueous layer, immiscible = organic layer) is the standard, rapid
identification method used in organic chemistry laboratory practice.
Q4: A student performing vacuum filtration using a Büchner funnel forgets to clamp the filter flask
before turning on the water aspirator. What is the most likely safety hazard that will result?
A. The filter flask will tip over, potentially spilling the filtrate and breaking the apparatus
B. The vacuum will be insufficient to draw the solvent through the filter paper
C. The water aspirator will back-siphon into the filter flask, contaminating the product
D. The filter paper will tear under the pressure of the unclamped flask
Correct Answer: A
Rationale: Correct because the water aspirator generates significant vacuum force; an unclamped
filter flask is unstable and will likely tip over, causing glass breakage, solvent spillage, and
potential exposure to hazardous materials. Per organic chemistry lab standard, the flask must
always be clamped securely.
Q5: During thin-layer chromatography (TLC) analysis of a reaction mixture, a student spots the
sample too heavily on the plate. What is the most likely consequence observed after
development?
A. The Rf values will be artificially elevated due to overloading
B. The spots will appear streaked or elongated, making Rf determination inaccurate
C. The solvent front will not migrate properly due to spot interference
D. The spots will be invisible under UV light visualization
Correct Answer: B
Rationale: Correct because overloading the TLC plate causes sample spreading and streaking as
the compound cannot partition properly between the stationary and mobile phases, resulting in
elongated spots that prevent accurate Rf measurement and visualization.
Q6: A student is selecting a recrystallization solvent for a compound that is soluble in hot
methanol but too soluble in cold methanol. Which solvent modification strategy should be
employed?
A. Use methanol as the sole solvent and cool the solution more rapidly
B. Use a solvent pair by adding water (a poor solvent) to the hot methanol solution until turbidity
appears, then reheating to clarify
C. Switch to a completely different solvent such as hexane
D. Increase the concentration of the compound in the methanol solution
Correct Answer: B
Rationale: Correct because when a compound is too soluble in a single solvent at all
temperatures, a solvent pair (good solvent + poor solvent) is used; water is added to the hot
methanol until slight turbidity indicates saturation, then reheated to clarify, allowing controlled
crystallization upon cooling.
Q7: In a fractional distillation separating a mixture of two liquids with a boiling point difference of
25°C, which factor most significantly improves the separation efficiency?
Chemistry Lab CA3 Answer Key | 2026
Update | 100% Correct.
SECTION 1: Laboratory Techniques & Safety (Questions 1–8)
Q1: During recrystallization of a crude organic solid, a student observes that the compound
dissolves completely in hot ethanol but does not crystallize upon cooling to room temperature.
Which step should be performed next to induce crystal formation?
A. Add excess ethanol and reheat to dissolve any remaining solid
B. Add a small amount of seed crystals to the cooled solution
C. Increase the cooling rate by placing the flask in an ice bath
D. Filter the solution while hot to remove insoluble impurities
Correct Answer: B
Rationale: Correct because seeding provides a nucleation surface for crystal growth, inducing
crystallization in a supersaturated solution that fails to crystallize spontaneously. Per organic
chemistry lab standard, seeding is the preferred first step before more aggressive cooling
methods.
Q2: A student is performing a simple distillation to purify a liquid with a boiling point of 156°C.
The thermometer is placed in the distilling flask rather than at the side arm of the still head. What
is the primary consequence of this error?
A. The distillate will be collected at a lower temperature than the true boiling point
B. The distillate will be collected at a higher temperature than the true boiling point
C. The thermometer will measure the temperature of the liquid, not the vapor, yielding inaccurate
boiling point data
D. The distillation will proceed faster due to better thermal contact
Correct Answer: C
Rationale: Correct because the thermometer must be positioned at the side arm to measure the
temperature of the vapor in equilibrium with the condensing liquid; placement in the flask
measures liquid temperature, which is not representative of the boiling point and yields
inaccurate data.
Q3: In a liquid-liquid extraction using a separatory funnel, a student is unsure which layer is the
organic phase. Which test should be performed to determine the identity of the layers?
A. Add a few drops of water to the funnel and observe which layer the drops join
B. Measure the density of each layer using a hydrometer
C. Add a small amount of sodium chloride to each layer and observe solubility
D. Remove a small sample of each layer and test with litmus paper
Correct Answer: A
, Rationale: Correct because adding water droplets to the separatory funnel and observing which
layer they join (miscible = aqueous layer, immiscible = organic layer) is the standard, rapid
identification method used in organic chemistry laboratory practice.
Q4: A student performing vacuum filtration using a Büchner funnel forgets to clamp the filter flask
before turning on the water aspirator. What is the most likely safety hazard that will result?
A. The filter flask will tip over, potentially spilling the filtrate and breaking the apparatus
B. The vacuum will be insufficient to draw the solvent through the filter paper
C. The water aspirator will back-siphon into the filter flask, contaminating the product
D. The filter paper will tear under the pressure of the unclamped flask
Correct Answer: A
Rationale: Correct because the water aspirator generates significant vacuum force; an unclamped
filter flask is unstable and will likely tip over, causing glass breakage, solvent spillage, and
potential exposure to hazardous materials. Per organic chemistry lab standard, the flask must
always be clamped securely.
Q5: During thin-layer chromatography (TLC) analysis of a reaction mixture, a student spots the
sample too heavily on the plate. What is the most likely consequence observed after
development?
A. The Rf values will be artificially elevated due to overloading
B. The spots will appear streaked or elongated, making Rf determination inaccurate
C. The solvent front will not migrate properly due to spot interference
D. The spots will be invisible under UV light visualization
Correct Answer: B
Rationale: Correct because overloading the TLC plate causes sample spreading and streaking as
the compound cannot partition properly between the stationary and mobile phases, resulting in
elongated spots that prevent accurate Rf measurement and visualization.
Q6: A student is selecting a recrystallization solvent for a compound that is soluble in hot
methanol but too soluble in cold methanol. Which solvent modification strategy should be
employed?
A. Use methanol as the sole solvent and cool the solution more rapidly
B. Use a solvent pair by adding water (a poor solvent) to the hot methanol solution until turbidity
appears, then reheating to clarify
C. Switch to a completely different solvent such as hexane
D. Increase the concentration of the compound in the methanol solution
Correct Answer: B
Rationale: Correct because when a compound is too soluble in a single solvent at all
temperatures, a solvent pair (good solvent + poor solvent) is used; water is added to the hot
methanol until slight turbidity indicates saturation, then reheated to clarify, allowing controlled
crystallization upon cooling.
Q7: In a fractional distillation separating a mixture of two liquids with a boiling point difference of
25°C, which factor most significantly improves the separation efficiency?
