Question

An unknown compound melting at 131 - 133 C. It is thought to be one of the following compounds: trans-cinnamic acid (133-134); benzamide (128-130); DL-malic acid (131 - 133); or benzoin (135-137). The mixture of melting points of the unknown compound with each of the test compounds listed below. What is the unknown compound? trans-cinnamic acid MP: 110-120 Benzamide MP:130-132 Dl-malic acid MP: 114-124 Benzoin 108-116

Answer 1

Answer:

benzamide

Explanation:

Compound            melting Point ,ºC          Melting Pont Mixture, ºC

       X                          131 - 133

trans-cinnamic            133 - 134                      110 - 120

acid

benzamide                 128 - 130                       130-132

malic acid                   131   -133                        114 -124

Benzoin                      135 - 137                        108 - 116

The compound X is benzamide since the melting point range is the one closest to this compound (  130-132 ºC)

The reason there is not an exact match is not due due to the presence of impurities. The presence of impurities always lower the melting point ( it is a coligative property such as the melting point depresion of salt and water )

The reason for the deviation must be  be some other factors such as preparation of the sample in the capillary, errors in reading the thermometer, rate of heating, etc.