Organic Chemistry 12A, Lab Exam 2026
Questions and Answers
If a crystalline substance is pure, its melting point should - Correct answer-Occur
over a small temperature range, ca. 1 degree C.
Prior to loading into the capillary melting point tube, a crystalline sample should
be finely pulverized - Correct answer-- In order to be easily compacted at the
bottom of the tube.
- In order to maximize the surface area of the crystals so they react faster
Impurities in a crystalline sample generally cause the melting point range to be -
Correct answer-Low and Broad
If a melting point sample is loosly packed in the bottom of the capillary tube, in
other words, there are air spaces between the particles, the melting point range will
probably be - Correct answer-high and broad.
If an impure solid is recrystallized, the melting point range of the recrystallized
solid should be - Correct answer-higher and narrower than that of the impure solid.
©COPYRIGHT 2025, ALL RIGHTS RESERVED 1
,Selection of an appropriate recrystallization solvent is extremely important. In a
good recrystallization solvent, the solute you wish to recrystallize will be "soluble
_____, insoluble _____." - Correct answer-hot, cold
Which of the following is NOT a characteristic or quality of a good
recrystallization solvent? - Correct answer-cannot be miscible with water
Why is it necessary to cool a hot, supersaturated solution SLOWLY? - Correct
answer-rapid cooling will cause precipitation thereby trapping impurities in the
crystals
One of the most commonly asked questions in O-chem lab is "How much solvent
should I use for my recrystallization?" The answer is always the same: - Correct
answer-use the minimum amount
When preparing to vacuum filter your crystals, it is important to remember this
when you set up: - Correct answer-- clamp the vacuum flask securely so it does not
tip over
- use the black, heavy-walled tubing, not the amber, thin-walled tubing when
connecting the filter flask to the vacuum source
- only use filter paper that is the appropriate size for your Buchner funnel
©COPYRIGHT 2025, ALL RIGHTS RESERVED 2
,It is common for some crystals to remain in the crystallization flask after you have
poured out the crystal slurry onto the filter paper. You should... - Correct answer-
wash the crystals from the flask using a small amount of ice-cold recrystallization
solvent
We learned in General Chemistry that water is an excellent solvent for many ionic
inorganic compounds. Consequently, water, because of its highly polar nature,
should never be used as a recrystallization solvent for nonionic organic
compounds. - Correct answer-- False
- Water is highly polar, and it often makes a good recrystallization solvent for
organic compounds with highly polar, hydrogen-bond-capable organic molecules.
Examples of such molecules would be sugars (mono- or di-saccharides like
glucose and sucrose) or some carboxylic acids, including amino acids. In this
experiment, we will recrystallize benzoic acid and acetanilide from water.
Which of the following is NOT a technique that would induce crystallization? -
Correct answer-- Add more solvent
After filtration, crystals should be left on the filter paper with the vacuum on for at
least 10 minutes. Otherwise, - Correct answer-- the melting point of the crystals
may be too low and broad - the melting point of the crystals may be too high
©COPYRIGHT 2025, ALL RIGHTS RESERVED 3
, - the mass of the crystals may be artifically high - (two of these)
You look up the melting point of the amino acid serine and find that it is listed as
"228d." What does that reported melting point mean? - Correct answer-"d" stands
for decomposition
Never heat a closed system, otherwise it may ____________. - Correct answer-
explode
The _______________ boiling point of a liquid is measured at 760 torr (1 atm). -
Correct answer-normal
A liquid will boil when its equilibrium vapor pressure equals the total (ambient)
pressure. We can tell the boiling process has begun - Correct answer-when the
liquid begins to bubble
Imagine a pure liquid containing a soluble but nonvolatile "impurity." Examples
might be salt water, or some polyethylene molecules dissolved in hexane. The
presence of nonvolatile "impurities" will have no effect on the boiling point of the
liquid. - Correct answer-False
The presence of a volatile impurity in a liquid (like a little water in some acetone)
will have no effect on the boiling point of the liquid. - Correct answer-False
©COPYRIGHT 2025, ALL RIGHTS RESERVED 4
Questions and Answers
If a crystalline substance is pure, its melting point should - Correct answer-Occur
over a small temperature range, ca. 1 degree C.
Prior to loading into the capillary melting point tube, a crystalline sample should
be finely pulverized - Correct answer-- In order to be easily compacted at the
bottom of the tube.
- In order to maximize the surface area of the crystals so they react faster
Impurities in a crystalline sample generally cause the melting point range to be -
Correct answer-Low and Broad
If a melting point sample is loosly packed in the bottom of the capillary tube, in
other words, there are air spaces between the particles, the melting point range will
probably be - Correct answer-high and broad.
If an impure solid is recrystallized, the melting point range of the recrystallized
solid should be - Correct answer-higher and narrower than that of the impure solid.
©COPYRIGHT 2025, ALL RIGHTS RESERVED 1
,Selection of an appropriate recrystallization solvent is extremely important. In a
good recrystallization solvent, the solute you wish to recrystallize will be "soluble
_____, insoluble _____." - Correct answer-hot, cold
Which of the following is NOT a characteristic or quality of a good
recrystallization solvent? - Correct answer-cannot be miscible with water
Why is it necessary to cool a hot, supersaturated solution SLOWLY? - Correct
answer-rapid cooling will cause precipitation thereby trapping impurities in the
crystals
One of the most commonly asked questions in O-chem lab is "How much solvent
should I use for my recrystallization?" The answer is always the same: - Correct
answer-use the minimum amount
When preparing to vacuum filter your crystals, it is important to remember this
when you set up: - Correct answer-- clamp the vacuum flask securely so it does not
tip over
- use the black, heavy-walled tubing, not the amber, thin-walled tubing when
connecting the filter flask to the vacuum source
- only use filter paper that is the appropriate size for your Buchner funnel
©COPYRIGHT 2025, ALL RIGHTS RESERVED 2
,It is common for some crystals to remain in the crystallization flask after you have
poured out the crystal slurry onto the filter paper. You should... - Correct answer-
wash the crystals from the flask using a small amount of ice-cold recrystallization
solvent
We learned in General Chemistry that water is an excellent solvent for many ionic
inorganic compounds. Consequently, water, because of its highly polar nature,
should never be used as a recrystallization solvent for nonionic organic
compounds. - Correct answer-- False
- Water is highly polar, and it often makes a good recrystallization solvent for
organic compounds with highly polar, hydrogen-bond-capable organic molecules.
Examples of such molecules would be sugars (mono- or di-saccharides like
glucose and sucrose) or some carboxylic acids, including amino acids. In this
experiment, we will recrystallize benzoic acid and acetanilide from water.
Which of the following is NOT a technique that would induce crystallization? -
Correct answer-- Add more solvent
After filtration, crystals should be left on the filter paper with the vacuum on for at
least 10 minutes. Otherwise, - Correct answer-- the melting point of the crystals
may be too low and broad - the melting point of the crystals may be too high
©COPYRIGHT 2025, ALL RIGHTS RESERVED 3
, - the mass of the crystals may be artifically high - (two of these)
You look up the melting point of the amino acid serine and find that it is listed as
"228d." What does that reported melting point mean? - Correct answer-"d" stands
for decomposition
Never heat a closed system, otherwise it may ____________. - Correct answer-
explode
The _______________ boiling point of a liquid is measured at 760 torr (1 atm). -
Correct answer-normal
A liquid will boil when its equilibrium vapor pressure equals the total (ambient)
pressure. We can tell the boiling process has begun - Correct answer-when the
liquid begins to bubble
Imagine a pure liquid containing a soluble but nonvolatile "impurity." Examples
might be salt water, or some polyethylene molecules dissolved in hexane. The
presence of nonvolatile "impurities" will have no effect on the boiling point of the
liquid. - Correct answer-False
The presence of a volatile impurity in a liquid (like a little water in some acetone)
will have no effect on the boiling point of the liquid. - Correct answer-False
©COPYRIGHT 2025, ALL RIGHTS RESERVED 4
