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CHM2210L Midterm Exam Practice Questions and Answers 100% Pass Do all mixtures of two different organic compounds exhibit a broad melting point range? - ANSWER- No, eutectic mixtures(minimum mp for a mixture) melt with a sharp mp range. Do organic compounds that exhibit polymorphism have differ...

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CHM2210L Midterm Exam Practice

Questions and Answers 100% Pass


Do all mixtures of two different organic compounds exhibit a broad melting point range? - 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? - 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? - ANSWER✔✔-

Darkening, change in color, effervesence


What characteristic in melting point behavior does an impure organic compound usually exhibit? -

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? - 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?* - 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. - ANSWER✔✔-Some organic

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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 - 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? - 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.




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C- The sample is firmly packed into the capillary tube. - 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? -

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? - ANSWER✔✔-238°C-240°C


Define


Melting point


Mixed melting point


Eutectic point - ANSWER✔✔-- the range which in the compound begins and finishes melting.




-The point which a 50:50 mixture of A and B melt.


- The minimum melting point of a mixture


Explain how a eutectic mixture could be mistaken for a pure substance and comment on whether

encountering a eutectic mixture would be a common or uncommon encounter. - ANSWER✔✔-A eutectic


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