January 17, 2017

Homework Help: Same substances?

Posted by Sheryl on Sunday, October 1, 2006 at 7:50pm.

Question: Two substances, A and B, have the same melting point. how can you determine if they are the same without using any form of spectroscopy? Explain in detail.

My answer: Perform a mixture melting point. In this procedure, pulverize and mix A and B in equal quantities.. Then determine the melting point. If there is a melting point depression or if the range of melting is expanded by a large amount, you may conclude that one compound has acted as an impurity toward the other and that they are not the same compound.

Is this okay. Didn't know if it would work for two substances with the same temperature.


Yes, that's when it DOES work.
Remember delta T = kf x m
So if we have the same substance, then adding the two 50-50 produces no change, and delta T doesn't change because the melting point isn't depressed. But if the stubstances are not the same, then the molality changes, and delta T changes. In fact, if you know the molal freezing point depression and you know the amount of substances mixed (both of them), measurement of delta T will provide a molar mass. In this case, of course, since the identity of both A and B are not known, you would be required to use camphor or some other material and determine the molar mass of each separately.

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