Question

30pts total) 1. Irwo known substances have the same melting point as an unknown substance that you obtained, how could you determine if either of the two known substances is your unknown compound? 2. You are given an impure mix of substances that contains a solid substance, Chemical X, that is soluble in ethanol at all temperatures. You know there is an insoluble impurity in the mix as well as a very soluble impurity, both in ethanol. Can you recrystallize Chemical X in ethanol? Explain. 3. An unknown substance is placed upon a TLC plate. The plate is run in a nonpolar solvent and then placed under a UV light. The unknown substance moved up the plate only 1 cm to give a Rf value of 0.1 and there was only one spot. What can an experimenter determine about the purity and polarity of the unknown substance from these results? BI U

Answer 1

Answer to Question No.1 :

If two known substances have the same melting point as unknown substances, then the easiest way to check on the TLC plate. If both compounds are the same then Rf value will be the same; or Both are different then Rf value will be different.

Another way is the mixed melting point method: In which You should take three melting point capillaries, and record their melting point.

Capillary 1: compound 1

Capillary 2: compound 2

Capillary 3: Mixture of both Compound 1 and 2.

If the Result of capillary 3 matches with compound 1 and 2, then both compound 1 and 2 are the same. If the results differ then both are different compounds.

Answer to Question No.2 :

The answer is YES.

Here is the answer:

As mentioned in the question, compound X has two impurities. First, dissolve the compound in ethanol and filter it, so the Insoluble impurity will be separated out by simple filtration. Now if you perform the crystallization carefully then first compound X will be separated out and SOLUBLE impurity remains in the mother liquor. You get the pure compound.

Answer to Question No.3 :

As mentioned in case study; TLC was a run-in NON POLAR solvent system and Rf is only 0.1.

Purity: Inspite of getting single spot compound did not mean that it was pure. As Rf was only 0.1 then you should check the TLC by adding some polar solvent. so final answer is NO, It was not pure.

Polarity: It was a Polar substance. Because Polar compound binds with silica gel of TLC plate.