midterm Flashcards
(40 cards)
if one wishes to repeat the experiment it must be done with a fresh sample , why?
A. when cooled, compounds can change to their mirror image
B. compounds may decompose during melting
C. when cooled may form a different crystal structure
D. when melted compounds form different compounds
compounds may decompose during melting
when cooled may form a different crystal structure
what is the purpose of determining melting point in an organic lab
A. to determine purity of a compound
B. to identify a compound
C. to calculate heat of fusion
D. to determine functional group of an organic compound
to determine purity of a compound
to identify a compound
which of the following does not indicate the start of a melt
A. shrinking of sample
B. Discoloration
C. decomposition
D. sample sweating
all of them
What factors affect the melting point? Mark all that apply.
A. The intermolecular forces
B. mass of sample
C. volume of sample
D. size and shape of molecule
A and D
T or F
it is impossible to raise the MP of a pure substance
False
How is the mix melting point used to prove the identity of a compound? Mark all that apply.
A. Mixtures of two different compounds have higher melting points than for each individual compound
B. Melting point of the mixture will be the same as that for each individual compound if they are the same
C. Mixtures of two different compounds have lower melting points than for each individual compound
D. Mixtures of two of the same compounds have a broad melting range
B, C
What will happen during MP determination if air is trapped inside sample
A. It takes longer for the sample to melt and may exhibit a mistakenly sharp range of melting
B. Start of the melt may be hard to determine with a loosely packed sample
C. The air is not a good conductor of heat, so the heat will not be evenly distributed
D. It takes longer for the sample to melt and may exhibit a mistakenly broad range of melting
B, C, D
How does the presence of impurity affect a substance’s melting range?
A. The melting point is increased
B. The melting point is depressed
C. The melting range is broadened
D. The melting range is sharpened
B and C
True or False? A hydrate melts at a different temperature than the anhydrous compound.
true
True or False? A racemate behaves as a pure compound, with its own sharp melting point.
True
Which of the following can be used to collect distillate?
A. A graduated clinder when a certain volume of distillate is desired
B. A beaker submerged in an ice bath when the boiling point of distillate is low
C. A graduated cylinder when the boiling point of distillate is low
D. A boiling flask submerged in an ice bath when the boiling point of distillate is low
A and D
What is the purpose of using boiling chips in distillation?
A. provides a nucleation surface for the formation of bubbles which ensures even
boiling
B. provides a nucleation surface for the formation of bubbles which ensures law
boiling point to avoid decomposition of sample
C. provides a nucleation, surface for the formation of bubbles which prevents
bumping
D. provides a nucleation surface for the formation of bubbles which ensures fast boiling
A and C
A student forgot to put boiling stones while performing a distillation and started heating the liquid. Suddenly, he remembered that he forgot to do so. What should he do?
A. continue with the distillation
B. lower the heat and continue with the distillation
C. cool the liquid and add boiling stones
D. add boiling stones to the hot liquid
C
Why should the rate of heating be adjusted to 1 drop every 2 seconds, and not higher?
A. A faster distillation rate results in mixed vapor of all liquids reaching the thermometer and so results in an inaccurate boiling point reading.
B. A slower distillation results in not enough vapor reaching the thermometer to give an accurate boiling point reading
C. A faster distillation rate results in uncondensed vapor being carried through the condenser and into the room
D. A faster distillation rate results in poor separation of components.
all
Why are the glass joints of distillation apparatus greased?
A. to provide a good seal between joints
B. to prevent the glass joints from reacting together
C. to prevent the glass joints from freezing together
D. to allow glass joints to fit together
A and C
The literature reports a boiling point for cyclohexanol of 161°C. In Boulder, Colorado, the atmospheric pressure is usually about 625 mm Hg. Would the observed boiling point of cyclohexanol be higher or lower than 161? Why?
A. lower because atmospheric pressure is higher
B. higher because atmospheric pressure is low
C. higher because atmospheric pressure is higher
D. lower because atmospherie pressure is low
D
Which of the following compound pairs could be separated by simple distillation
A. hexanes and toluene
B. butyl acetate and butanol
C. acetone and aniline
D. cyclohexane and cyclohexanol
B and D
How does a fractionating column increases the effectiveness of distillation?
A fractionating column allows for many successive distillations to take place at once.
A fractionating column enriches the distillate in the lower bailing component by returning the liquid with a higher boiling point to the boiling flask
A fractionating column enriches the distillate in the lower bailing component by
filtering the liquid with a higher boiling point.
A fractionating column allows for boiling of all liquids to take place at once
A and B
True or False? The heating mantle is designed to work at 110 volts, so it is okay to connect it to a wall outlet
false
Why should liquid not exceed 2/3 of boiling flask volume, nor less than 1/3? Mark all that apply.
If liquid is less than 1/3 of boiling flask volume, a substantial amount of distillate
will be lost as vapors filling the flask at the end of the distillation.
If liquid is less than 1/3 of boiling flask volume, the liquid will boil at a lower temperature, so a wrong boiling point will be recorded.
If liquid exceeds 2/3 of boiling flask volume, boiling material may boil up into the distillation head.
If liquid exceeds 2/3 of boiling flask volume, the liquid will boil at a higher
temperature, so a wrong boiling point will be recorded.
A and C
True or False? For each recrystallization step, some of the desired compound will remain in solution.
true
How can a sample’s melting point indicate the presence of an organic impurity?
The melting range is broadened
The melting point is elevated
The melting point is depressed
The melting range is sharpened
A and C
During a recrystallization, which of the following will cause you to lose crystals?
Washing the crystals with minimum of cold solvent
Dissolving sample in minimum of hot solvent
Washing the crystals with solvent at room temperature
Dissolving sample in minimum of solvent at room temperature
C and D
What is the purpose of using boiling chips in recrystallization?
provides a nucleation surface for the formation of bubbles which ensures even
boiling
provides a nucleation surface for the formation of bubbles which prevents
bumping
provides a nucleation surface for the formation of bubbles which ensures fast boiling
provides a nucleation surface for the formation of bubbles which ensures fast dissolving of the sample
A and B
