Chemistry High School

Johnny is very careless in the lab. He purified two white solids (A and B) by re-crystallization, but forgot to label the vials. Both A and B have a melting point range of 10² - 10⁴ °C, so he is not sure which solid is which. He has a small amount of an authentic sample of pure A available in the lab. What should he do?

Answers

Answer 1

Answer:

The solution are in the explanation below

Explanation:

- Find the melting point of authentic sample of pure A.

- Mix sample A into both vials.

- Use melting point depression

- Lower melting component will liquefy first, and melting point will lower/broaden the range. (Determine which vial holds sample B mixed with A)

- Vial with the same sample A melting point range would remain consistent to 102-104˚C.

Answer 2

Johnny can identify the unknown samples A and B by conducting a melting point analysis with a mixture of the unknown and a known authentic sample of A. A unchanged melting point indicates the unknown is A, while a changed range suggests it is B. Proper technique is necessary for accurate results.

Johnny should perform a melting point analysis to determine which vial contains which substance. He can take a small amount of the authentic sample of pure A and mix it with a small amount of the unknown substance. He should then measure the melting point range of the mixture. If the mixture has a sharp, unchanged melting point range similar to the authentic sample's known value, it can be concluded that the unknown substance is also substance A. If the melting point range is depressed or broadened, it indicates the unknown substance is not A, but rather substance B.

It is important to note that if the expected melting point is not known, the substance should be heated at a medium rate to determine an approximate melting point. A second attempt should be made with a fresh sample for accuracy. Additionally, ensuring that the amount of solvent is suitable for the volume of the flask, and that heat is properly trapped can guarantee optimal re-crystallization conditions.

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