Lab Report: Cryoscopic Determination Of Molecular Weight

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Experiment 3: Cryoscopic Determination of Molecular Weight Aim: The primary objective of the experiment is to calculate the total molecular mass of the compound benzoic acid in a predetermined amount of cyclohexane. Introduction: One of the key concepts in this experiment is the term freezing point depression. The definition of this phrase is that when a solute is dissolved into a solvent, their collective boiling point will be less than that of the original boiling point of the solvent. This incidentally leads to another important definition, the colligative properties of a solution. This means a particular set of characteristics that are unique to that solution. As the freezing point of the solution decreases, the solution then attains atmospheric…show more content…
Approximately 15-20 grams of cyclohexane was added to the freezing-point tube and its total mass was calculated so that the exact mass of cyclohexane could be determined. A large beaker that was considerably bigger than the freezing-point tube of cyclohexane had its mass noted. Two holes were placed in a cork which was an appropriate diameter for the freezing-point tube. The cork was then placed in the freezing point tube and its total mass was noted. A dry thermistor and a dry stirrer were inserted into the tube through the two holes in the cork. It was assured that both the thermistor and the stirrer were immersed in the cyclohexane in the tube. The big beaker was filled with a large amount of ice and deionised water. The solution of cyclohexane was continuously stirred until it reached a uniform temperature. The freezing point of cyclohexane was determined by recording the time it took for the cyclohexane to freeze. The solution was then removed from the ice and allowed to return to its original temperature. This course of action was repeated at least three times and the average value was taken for the freezing point of cyclohexane. The exact same routine was used, except approximately 0.1 gram of benzoic acid was added to the freezing point tube. The freezing point was calculated for the solution of the predetermined mass of cyclohexane and 0.1 grams of benzoic acid. Again, it was completed a minimum of three times and the average of these values was taken. This was repeated two more time times and the mean freezing point values for each of these measurements were determined. This meant that there was data for at least three solutions as well as that of the pure

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