lab-7postlab - there is a constant volume then the freezing...

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Sam Eichenblatt Post Lab 7 A2 Jeff Culiver The goal of this experiment was to find the molecular weight of an unknown by measuring the change in freezing point when the substance is added into water. The change in temperature is related to the molecular weight by the equation DeltaT=Kw*m. Where Kw is a constant for water and m is molality which is moles/Kg of solvent. Using moles=mass/molecular weight we can substitute and have the final formula of: DeltaT=Kw((mass/molecular weight)/KgSolvent). Now there is a direct relationship between change in freezing point and molecular weight. The change in freezing point was not constant so we took the average of the two changes in temperatures. The calculations eventually ended up being closest to the molecular weight of Urea. Adding this substance to the water causes the freezing temperature to decrease. This is because of intermolecular interactions. Dipole-ionic interactions force the urea to disassociate in the water thus making the water harder to freeze together. The change in freezing point is directly related to the number of molality, and if
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Unformatted text preview: there is a constant volume then the freezing point is related to number of moles. So the higher the ratio of moles to kg of solvent the greater the change in temperature. The change in freezing point was averaged to be 3.03 C. This correlated to the molecular weight of 32.74. This molecular weight is not close to any of the molecular weights listed in the table of possible unknowns. This is because for one of our trials our solution never froze, it only supercooled. This caused the change in temperature to be much higher than it actually was and forced the molecular weight to be much lower than the actual weight. Other errors could have been poorly calibrated equipment, poorly measured water or unknown weight, or human error in calculations. This experiment forced the student to understand the parts of the change in temperature equation and expanded his or her knowledge in freezing and boiling points...
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