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CH 118 molecular weight by freezing point

CH 118 molecular weight by freezing point - very close then...

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Molecular Weight by Freezing Point I. Molecular Weight of the Unknown The molecular weight of unknown #388 is 163.345 g/mol ± 0.05. II. Determination of Kf of Water A similar procedure could be used to determine the Kf of water. A crystal substance whose Kf is already known can be used with water. The mass and moles of water and the substance used can be noted and the molality calculated by using the formula molality = moles of solute/ Kg of solution. The water and substance can be heated the crystals melt and the temperature noted. Then, the water and substance can be cooled until crystals appear again and note the temperature. These temperatures are used to calculate ∆Tf. The formula Kf = ∆Tf / molality can then be used to calculate Kf. III. Properties of New Solvent If I were to select a different solvent for this experiment I would consider factors such as its freezing and melting points. If there was a large difference between these two then it may take a long time to heat it and then cool it down. Also, if the freezing and melting points were
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Unformatted text preview: very close then it the experiment may happen too quickly and the temperatures harder to determine exactly. I would also consider the solvent’s molecular weight because too large or small numbers could make calculations more difficult and less accurate. IV. Using Freezing Point to Assess Purity If a substance is believed to have a certain identity then its freezing point depression can be determined experimentally. This experimentally determined freezing point can be compared to the accepted value of the freezing point depression of the pure substance and if it is very close then it can be considered pure. V. Limitations of this Procedure If the actual molecular weight was 75 g/mol it would be difficult to calculate the molecular weight of a compound. This number would be close to the difference of the temperatures and the molality and cause very small numbers....
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