Cyclohexane Lab Report

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PChem Experiment No. 3
Cryoscopic determination of molecular weight
1. Aims
To determine the apparent molecular weight of benzoic acid in cyclohexane by creating a binary system and finding the different freezing points of the solutions.

3. Experimental Procedure
A dry, empty freezing-point tube and its cork were weighed together. Carefully, 15-20g of pre-cooled cyclohexane was poured into the freezing-point tube, the tube recorked, and the whole thing was reweighed, to find the exact mass of the cyclohexane. The dry thermistor and stirrer were inserted into the FP tube, ensuring they were immersed in the liquid. A large beaker was filled with ice and water, which the FP tube was placed into. The cooling curve was determined by recording the temperature at regular time intervals (every fifteen seconds) as the cyclohexane cooled, until the temperature became constant. The cyclohexane was stirred continuously. The FP tube was then removed from the ice and the cyclohexane melted. The cooling of the cyclohexane was then repeated twice and the mean of the three values was taken as the freezing point.
~0.1g of benzoic acid was accurately weighed and added to the cyclohexane and stirred vigorously until it had dissolved. The freezing point was then determined as before, the solution melted
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Solids are crystalline and are generally only composed of a single chemical species. Because they are ordered and are not mixtures, their entropy is very low compared to the entropy of a liquid. However, because solids have a lower enthalpy than liquids a competition between entropy and enthalpy sets the freezing point of a solid. If the liquid is a mixture, it will have a higher entropy than a pure liquid and hence, the entropy loss due to freezing will be greater. This greater loss of entropy leads to a lower freezing

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