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Question 14:
The melting point of a pure compound is known to be 110-111ºC. Describe the melting behavior expected if this compound is contaminated with 5% of an impurity.

A. 90-96ºC
B. 111-113ºC
C. 90-91ºC
D. 110-111ºC

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Question 15:
You and your lab partner take melting points of the same sample. You observe a melting point of 101-107°C, while your partner observes a value of 110-112°C. Explain how you can get two different values with exactly the same sample.

A. One of the lab partners may have heated the melting point device too rapidly.
B. All of the answers are correct.
C. Different people can make different judgments as to when a compound has begun to melt.
D. Melting point apparatus can be out of calibration.

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Question 16:
For which molecule below, was the melting point reported wrong by a student?

Observed Melting Points:
- Compound A: Naphthalene, 79-80ºC
- Compound B: Benzophenone, 45-47ºC
- Compound C: D-Anisic acid, 178-182ºC
- Compound D: 3-Chlorobenzoic acid, 157ºC

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Question 17:
A pure compound melts at 48-49ºC. If a trace amount of an impure compound, whose melting point is 80-82ºC, is mixed with the first compound, what happens to the melting point of the pure compound?

A. It is impossible to determine without knowing the identity of the two compounds.
B. The second compound would raise the melting point of the first one.
C. The second compound would lower the melting point of the first one.
D. The second compound would have no effect on the melting point of the first one.

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Question 18:
You have an unknown compound with an observed melting point of 90-93ºC (not pure, as the range is wide). Which compound (Letter) is your compound by checking the known compounds below (reported melting points are given)?

A. 95.5-96ºC
B. 90.5-91ºC
C. 88-90ºC
D. Hard to know

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Question 19:
In the melting point-composition diagram above, figure out the eutectic ratio and temperature for the solid mixture of CHCl3 and CCl4.

A. 0.5 and 192 K
B. 0.6 and 200 K
C. 0.4 and 190 K
D. 0.5 and 190 K

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Which of the following is not correctly paired with its dominant type of intermolecular forces?

A. NH3, hydrogen bonding
B. HBr, hydrogen bonding
C. CaO, ionic forces
D. C6H6 (benzene), Van der Waals

Answer :

Question 14The melting point of a pure compound is known to be 110-111º. If this compound is contaminated with 5% of an impurity, the melting behavior expected is: 110-111º. Impurities will cause melting points to be depressed and the melting range to be broadened, thus the melting point will still be the same.

Question 15Different people can make different judgments as to when a compound has begun to melt, and the melting point apparatus can be out of calibration. This is why both lab partners get two different values with exactly the same sample.

Question 16For the molecule, D-Anisic acid the reported melting point was wrong by the student.

Question 17If a trace amount of an impure compound, whose melting point is 80-82 degrees Celsius, is mixed in with the first compound, the second compound would lower the melting point of the first one.

Question 18The unknown compound with an observed melting point of 90-93 is Compound D.

Question 19The eutectic ratio and temperature for the solid mixture of CHCl3 and CCI4 is 0.5 and 190K respectively. The pairing of Cao with ionic forces is not correct.

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