Question

Recrystallization: Percent error of melting point and how impurities affect melting point:

Please explain both questions below:

(Part A) During a recrystallization experiment, the literature melting point of a compound is 105-107 °C. The melting point temperature you obtain from your recrystallized compound is 103-105 °C. Which of the following would be the proper way to set up the percent error of your mp? (a) (104/106)*100 (b) [(106-104)/106] *100 (c) (107/105)*100 (d) [(107-105)/107] *100 (Part B) For a recrystallization experiment (halohydrin reaction of butadiene sulfone with NBS in water solvent), if the product before recrystallization had a melting point of 184-188 °C but after recrystallization the product had a melting point of 176-186°C, which of the options below would be the best option and why? (a) The product would need to be dried some more, first by wicking it. Water was the recystallization solvent. Water in the product is acting as an impurity and is causing the melting point to be lower and broader after recrystallization. (b) The glassware used for recrystallization was either dirty or contaminated. Trying to purify the compound made it more impure impurities lower the melting point and increase the melting point range). Using a different solvent like acetone would rinse the impurity off the product when doing the recrystallization because acetone rinses organic products. (c) The melting point is satisfactory

Answer 1

Part A

Given, the melting point of a given compound according to literature is 1050-1070C

and observed melting of point of a given compound is 1030-1050C.

If we have to calculate the percent error then, we have to calculate the mean of standard mp temperature range and observed mp temperature range is (105+107)/2 =106 & (103+105)/2 =104 respectively.

Now, according to % error definition, % error can be expressed as; % error = [(absolute value - observed value)/absolute value] x 100%

=[(106-104)/106]x100% ----eqn(1)

this eqn is same as option "B" given in a question, there for option B is correct.

Part B

option (C): the product quality is satisfactory because the standard mp is 1840-1880C and observed mp is 1760-1860C. that means product mp shows the mp at standard mp range.