Imagine that you have a pure sample of an unknown compound that you are told is either 2-naphthol OR benzoic acid. In the lab, you also have access to samples of pure 2- naphthol and pure benzoic acid. However, both of these pure substances melt at 123 °C: the same temperature. Briefly describe how you could determine the identity of your unknown sample using melting points ONLY. HINT: impure substances have different melting points than pure substances.

I have no idea. I know that impure substances have lower melting points than pure substances, but that's all I know. How should I approach this problem? Thanks!

remember Molality? That determines the change in freezing point.

change freezing point=K*molessolute/moles solvent

First determine the freezing point depression for the solvents napthol, do this by experiment using a couple of solutes.
Then, knowing K, take the unknown let it be the solvent (add a known moles of one of the solvents you used in the first experiment, measure the new freezing point depression, then compute from the mass of the unknown solvent, how many moles you had, based on freezing pt depression. You know then if it is Napthaol or Benzoic acid.

That is great advice! Thanks, but the question asks specifically for using only melting point, so how do I do it with only melting point?

To identify the unknown compound using melting points only, you can perform a mixed melting point determination. Here's how you can approach the problem:

1. Take a small sample of the unknown compound and record its initial melting point using a melting point apparatus.

2. Next, take a small amount of pure 2-naphthol and mix it thoroughly with the unknown compound.

3. Repeat step 1 with the mixture of the unknown compound and pure 2-naphthol. Record the melting point of this mixture.

4. If the melting point of the mixture is the same as the melting point of pure 2-naphthol (123°C), then it is likely that the unknown compound is 2-naphthol.

5. If the melting point of the mixture is lower than 123°C, it suggests that the unknown compound is not pure 2-naphthol and may be benzoic acid.

6. To confirm this, repeat steps 2-5 using pure benzoic acid instead of pure 2-naphthol.

7. If the melting point of the mixture matches the melting point of pure benzoic acid (123°C), then the unknown compound is likely benzoic acid.

By comparing the melting points of the mixtures with pure substances, you can identify the unknown compound as either 2-naphthol or benzoic acid based on whether the unknown's melting point matches that of pure 2-naphthol or pure benzoic acid.

To determine the identity of the unknown compound using melting points, you can follow these steps:

1. Begin by obtaining the melting point of the unknown compound. You can do this using a melting point apparatus, which is a device specifically designed to measure melting points. Place a small quantity of the unknown compound in a capillary tube and insert it into the melting point apparatus.

2. Observe the melting point of the unknown compound. If the melting point matches exactly with the known melting point of either 2-naphthol or benzoic acid (123 °C in this case), it is not possible to directly confirm the identity of the unknown compound.

3. However, if the melting point of the unknown compound is lower than 123 °C, it suggests that the unknown compound is an impure mixture. Impurities typically lower the melting point of a substance. This would indicate that the unknown compound is not pure 2-naphthol or pure benzoic acid.

4. If the melting point of the unknown compound is significantly different from the known melting point of either 2-naphthol or benzoic acid, it indicates that the unknown compound is likely a different substance altogether and not a mixture of the two.

5. Now, to determine whether the unknown compound is 2-naphthol or benzoic acid, you can perform a melting point comparison using the pure samples. Measure the melting points of both pure 2-naphthol and pure benzoic acid using the same equipment and conditions.

6. If the melting point of the unknown compound is closer to the melting point of pure 2-naphthol, it suggests that the unknown compound is likely 2-naphthol. Similarly, if it is closer to the melting point of pure benzoic acid, it indicates that the unknown compound is likely benzoic acid.

It is important to note that melting point alone cannot provide definitive identification in this scenario. Further confirmation would be required, such as additional testing or comparison with other physical properties or analytical techniques.