An unknown sample is suspected to be one of the following compounds (true mp): u

Question

An unknown sample is suspected to be one of the following compounds (true mp): urea (133 C), 2-4 dimethylacetanilide (133 C), phenacetin (134 C) or 3- chlorobenzamide (134 C). A list of melting points when the unknown sample has been mixed 50% with each of the possibilities and taken on a Gallencamp mp machine is shown in the following table:

substance mixed with unknown melting point

urea 120-130

2-4 dimethylacetanilide 105-130

phenacetin 115-128

3-chlorobenzamide 133-134

Identify the unknown and explain

Explanation / Answer

The unknown must be 3-chlorobenzamide,as its true m.pt is (134C) that corresponds to the m.pt of the mixture(50% unknown+50% 3-chlorobenzamide).Also the range obtained is sharp that indicates presence of no impurity in the mixture.

explanation=Presence of impurity causes the m.pt to deviate from its true value rather lowers it.Also we get a melting range rather and not a sharp point due to impurities.It is so because impurities defect the crystal lattice of the compound,and the crystalline symmetry is disturbed.

Here in the experiment , the mixture(50% unknown+50% 3-chlorobenzamide) didnt show the presence of impurity as both the unknown and known were same compound,

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