CHM2210L Midterm UPDATED ACTUAL Exam Questions and CORRECT Answers
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Course
CHM2210L
Institution
CHM2210L
CHM2210L Midterm UPDATED ACTUAL
Exam Questions and CORRECT Answers
Do all mixtures of two different organic compounds exhibit a broad melting point range? -
CORRECT ANSWER- No, eutectic mixtures(minimum mp for a mixture) melt with a
sharp mp range.
Do organic compounds that exhibit polymorph...
CHM2210L Midterm UPDATED ACTUAL
Exam Questions and CORRECT Answers
Do all mixtures of two different organic compounds exhibit a broad melting point range? -
CORRECT ANSWER✔✔- No, eutectic mixtures(minimum mp for a mixture) melt with a
sharp mp range.
Do organic compounds that exhibit polymorphism have different melting points? -
CORRECT ANSWER✔✔- Yes, variations in the intermolecular attraction of different
crystalline forms can cause polymorphs to have different but sharp mp
What might you observe when an organic compound melts with decomposition? -
CORRECT ANSWER✔✔- Darkening, change in color, effervesence
What characteristic in melting point behavior does an impure organic compound usually
exhibit? - CORRECT ANSWER✔✔- A broad and depressed mp range
Given that your unknown was one of those listed on page 1-8, if you obtained a corrected
melting point range of 68-70°C, could you definitely identify what your unknown was? -
CORRECT ANSWER✔✔- According to the information, the unknown could either be
benzhydrol,phenyl benzoate, or biphenyl. After performing a mix-melting experiment than
you would be able to identify your unknown.
When should the start of the melt be recorded?* - CORRECT ANSWER✔✔- When the first
appearance of liquid within the bulk of the sample is seen.
A compound is observed to melt sharply at 111°C with the vigorous evolution of a gas; then
the compound solidified and did not melt until 155°C sharply. Explain. - CORRECT
ANSWER✔✔- Some organic compounds undergo 'elimination' reactions when heated,
losing such small molecules as H2O or CO2 and yielding other organic products. In this case,
A → B + gas
mp = 111d mp= 155
d= decomposition
, Suppose that your sample melts before you are ready to record the melting point. Should you
cool capillary and redetermine the melting point, or begin with a fresh sample? Explain -
CORRECT ANSWER✔✔- Start with a fresh sample; many organic compounds melt with at
least some decomposition- or when cooled, solidify in a different polymorphic form, and thus
would exhibit a different mp from that of the original sample.
Why is it not good practice to mix two components by simply stirring them together and then
sampling them for a mixed melting point? - CORRECT ANSWER✔✔- Stirring two solid
components together does not yield a homogenous mixture. A small sample for a mp
determination might thus be practically 100% of one component.
Give a reason for using the technique.
A- The rate of heating is adjusted to no more than 4° per minute when the solid is melting
B- The capillary tube is filled with no more than 2mm of sample.
C- The sample is firmly packed into the capillary tube. - CORRECT ANSWER✔✔- A-If the
temperature is increased too rapidly at the melting point, the thermometer, sample, and
heating block will not be at thermal equilibrium, and an erroneously depressed & wide mp
range will be observed.
B- Too much sample will take longer to melt and exhibit an erroneously large and higher
melting point range will be observed.
C- A sample that is loosely packed, makes it difficult to determine when the start of the melt
begins.
A student accidently broke some pieces of glass which fell into the sample. The student found
that the melting point for that sample was 187-8. Is it necessary for the student to take the
melting point again? - CORRECT ANSWER✔✔- No, insoluble impurities, such as glass,
have no effect on the mp.
According to the thermometer calibration curve on page 1-7, what is the corrected melting
point range of a sample that melts at 239-241°C? - CORRECT ANSWER✔✔- 238°C-240°C
Define
Melting point
Mixed melting point
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