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Molecular Mass by Freezing Point Depression Lab

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on 4 January 2013

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Transcript of Molecular Mass by Freezing Point Depression Lab

Molecular Mass by Freezing Point Depression Prelab Purpose Graphs Conclusion 1. To determine the molecular mass of an unknown substance by finding the freezing point of two solutions (BHT and para-dichlorobenzene, and BHT and the unknown. Group 1 Trial 1 The molecular mass of the unknown substance is 307 g/mol. Therefore, the substance is Steric Acid which has a molecular mass of 284 g/mol. Darlene D'Souza, Nithya Mitta, Ashwini Parchure, Avni Shah 2.
a.) If the thermometer reading was 1.4 degrees C too high there would be no affect on the molecular mass because the change would be the same.
b) If solvent spilled out the MM would be lower because there would be more calculated moles for the weighed solvent.
c) If solute spilled the MM would be higher because there would be a lower calculated mole value for the weighed solute.
d) If solution spilled there would be no effect because the ratio of solute to solvent would be the same. Procedure 1. Assemble an apparatus as shown using a hot plate.
2. Mass 8.00 g of BHT, an empty test tube, and the test tube with the BHT.
3. Heat the water until 90 degrees Celsius after the test tube is submerged.
4. Once the water is at 90 degrees Celsius remove the test tube. Stir the BHT continuously and record the temperature every 20 seconds.
5. Record temperatures until at least five values are constant (make note of when crystals form).
6. Mass 1.00 g of para-dichlorobenzene and add it to BHT.
7. Heat both until melted and homogenous.
8. Remove from the water; Continuously stir and Record temperature every 20 seconds until there are six values after crystallization.
9. Repeat steps 1-8.
10. Repeat steps 1-8 replacing para-dichlorobenzene with an unknown. DATA Group 2 Data Unknown #1 Questions 1. Colligative properties are properties of solutions that depend upon the number of solute particles to the number of solvent molecules in a solution. Colligative properties include the vapor pressure, osmotic pressure, depression of freezing point and elevation of boiling point.

2.Adding a solute affects the phase diagram of a pure substance as it raises the boiling point of a solvent and lowers the freezing point of the solvent as well. Questions Continued... 3. The least precise measurement in the experiment was the reading of the temperature. This limits the significant figures because the temperature readings were only rounded to one significant figure. If I subtracted or added the temperatures it would only have plus or minus 0.1 significant figure, but if it is subtracted by any other values that has two or more significant figures, then the answer would only come out to having two or more significant figures.

4.It is advantageous to choose a solvent that has a large value for kfp because there will be a greater change in the melting point.There will be a larger drop in the freezing point which will give you more significant figures which will make your measurements more precise and accurate.

5.The pure solvent shows a level horizontal curve as solidification occurs, but the curve for the solution slopes downward slightly becuase the solution freezes according to the amount of solvent (a colligative property). When the solution freezes, the concentration changes which will also change the slope of the solidification. Analytical balance has a plus or minus 0.01 gram precision and the thermometer has a plus or minus 0.1ºC precision which limits the overall precision. Experimental Sources of Error Discussion of Theory Colligative Properties Molality freezing point constant k fp Raoult's Law Freezing Point Depression Discussion of Theory (continued) MM = 307 g/mol Steric Acid (284 g/mol) percent error: 8.10% change in temp = molality x freezing point constant Group 1 Trial 2 and Group 2 Trial 1 Group 2 Trial 2 Calculations DATA Group 1 Data Data cont'd Data cont'd Group 2 Calculations
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