Cryoscopic Lab

Cryoscopic Lab - this by analyzing when the graph evened...

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2/6/08 Cryoscopic Determination of Molecular Weight Introduction In this lab, we determined the freezing point of a pure substance (naphthalene) by recording temperature and time data for the cooling of the molten substance. We then found the freezing point of naphthalene mixed with an unknown substance and compared the change in freezing point. With this change, and given the constant K for the naphthalene solvent, we solved for the molality of the solution, and therefore being able to determine the molecular weight of the unknown material. Chemical Responsibility We used Bunsen burners in this lab, which always need to be watched with extreme care. Naphthalene isn’t directly harmful to the body, but needs to be disposed of in the correct container. Report Sheet (attached) Sample Calculations Discussion of Results I determined the freezing point of naphthalene to be 75 degrees Celsius. I found
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Unformatted text preview: this by analyzing when the graph evened out for temperature vs. time. Crystals first started appearing at about 82 degrees Celsius. The freezing point of the naphthalene and unknown solution was about 69 degrees Celsius, yielding a freezing point depression of 6 degrees. Crystals first appeared at about 75 degrees Celsius. From these results, I calculated the molality of the unknown to be .869 m. The molar mass was solved to be 162.32 g/mole. There may be errors with my results due to how we timed the experiment. We did not have a stopwatch for the first test, but we did for the second. This may have significantly altered our results. Conclusion From this lab, we learned how to determine the molality and molar mass of an unknown using the change in freezing point found experimentally and the given constant. Mass was found by weighing the substances....
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This note was uploaded on 08/15/2008 for the course CHM U215 taught by Professor Lui during the Spring '08 term at Northeastern.

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