Molecular Weight by Freezing Point Depression

# Molecular Weight by - point of the substance with water dissolved into it Then by using the formula ΔT f = K f m the K f of water could be

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Roshni Sheth CH 118-M3 March 31, 2010 Molecular Weight by Freezing Point Depression 1. Molecular Weight of the Unknown The molecular weight of the unknown substance is 382.0 g/mol. The class average for the molecular weight of the unknown, which also acts as its accepted value, is 527.6 g/mol. The percent error is -27.6%. The experimental value is not accurate because of such a large percent error. The freezing point depression was 8.3 ºC. 2. Determination of the K f of Water To determine the K f of water, a similar experiment could be carried out. First, the freezing point of another substance, which would become the solvent, would have to be determined. This substance would have to be non-aqueous because water will be dissolved into the substance. Next, water would be dissolved into the substance and the new freezing point would be determined. The freezing point depression could then be determined by subtracting the freezing point of the substance by itself from the freezing

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Unformatted text preview: point of the substance with water dissolved into it. Then, by using the formula ΔT f = K f ( m ), the K f of water could be determined. 3. Properties of the New Solvent Properties to be considered are freezing point depression, polarity, and purity of the solvent. 4. Using Freezing Point Depression to Assess Purity If the sample is not pure, the freezing point depression will differ from the expected value. Any additional substance in the sample will alter the experimental freezing point that is determined. 5. Limitations of this Procedure If the actual molecular weight was around 750 g/mol, the experiment would take a much longer time because the substance would have to be melted and freeze again. The freezing point of such a substance would possibly be inaccurate because of the long waiting time....
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## This note was uploaded on 06/28/2011 for the course CH 118 taught by Professor March during the Spring '09 term at University of Alabama at Birmingham.

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Molecular Weight by - point of the substance with water dissolved into it Then by using the formula ΔT f = K f m the K f of water could be

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