Question

1. You are conducting a melting-point determination to identify a compound whose handbook melting-point range is 112 - 114 °C. Comment on the purity of the sample, and the possible identity of the substance, when your experimental melting-point range is : A. 114 -116 °C B. 94 - 102 °C I would C. 102 - 103 °C

Answer 1

The reported melting point range is 112 - 114^oC.

A. Observed melting point range is 114 - 116^oC: Organic substances can not be shown higher melting point than reported melting point. This is observed unless there is something wrong such as procedure or equipment or presonal error etc. Most commor error for higher melting point observation is fast heating the sample. Recommended heating rate should be 1^oC/minute to avoid such type of error.

B. Observed melting point range is 94 - 102^oC: Lower melting point and wider range observed due to impure sample. In this experiment melting of sample start at 94^oC and range is also higher (i.e. 8^oC). Hence this sample is more impure.

C.   Observed melting point range is 102 - 103^oC: In this experiment, melting point start at 102^oC, which is lower than handbook melting point, hence this sample is not pure. Smaller the range of melting temperatures, the higher the purity of the sample. To compare sample "B" and "C" sample C contain less impurities  than sample B.