Calculations:Avg. MP of pure Lauric Acid: (Trial 1 + Trial 2)= (44.22C+45.76C)= 44.99C220.750 g BA & 8.00 g LA: gmol: 0.750g BA x 1 mol BA = 0.0061415 mol BA122.12g BA= 6.14 x 10-3mol BAgKg: 8.00g LA x 1 Kg LA = 0.008 Kg LA1000g LAmolality (m) = mol of solute = 6.14 x 10-3mol BA= 0.7675 mKg of solvent 0.008 Kg LA= 0.768 mT = 44.99C – 43.17C = 1.82CKf= T= 1.82C = 2.369791667 C/mm0.768 m = 2.37 C/m1.50 g BA & 8.00 g LA: gmol: 1.50g BA x 1 mol BA = 0.012283 mol BA122.12g BA= 1.23 x 10-2mol BAgKg: 8.00g LA x 1 Kg LA = 0.008 Kg LA1000g LA
molality (m) = mol of solute = 1.23 x 10-2mol BA= 1.5375 mKg of solvent 0.008 Kg LA= 1.54 mT = 44.99C – 36.55C = 8.44C Kf= T= 8.44C = 5.480519481C/mm1.54 m= 5.48 C/mAverage Kf= (2.37C/m + 5.48C/m)= 3.925C/m2= 3.93C/mDiscussion/Conclusion:The lab enabled us to see how adding a solute to a substance (in this case, adding Benzoic Acid to Lauric Acid) causes the freezing point of that solution to lower, as we know from the colligative properties that adding impurities causes freezing point depression. From first measuring the freezing point of pure Lauric acid and later measuring the freezing point of both the impure Lauric acid samples, we were able to calculate the change in temperature from the pure to the impure samples. From this value and the molality of the solution, we could then figure out the Kfof the solvent, Lauric Acid. This could also have been done the opposite way around where we had to figure out what the molecular weight and therefore, what solute we were introducing into the experiment. This procedure is used in many day-to-day science labs where unknown substances interfere with freezing & boilingpoints. Human errors are inevitable in the lab area. However, some other errors that might be affecting the data collected include: when re-melting the Lauric acid solutions to remove the temperature probe, the mass of Lauric acid could have decreased due to residue on the probe; the temperature probe may not have been correctly calibrated as when you are stirring the Lauric acid, the probe hits on the sides of the test tube frequently; the flat areas on the graphs were hard to find, so the actual melting/freezing point of the Lauric acid from the graphs may have been interpreted wrong. Some of these sources of error are impossible to remove from the situation, however, more accurate measurements and graph interpretations could eliminate some of the possible errors experienced in the lab. If this lab were completed 100% accurately, the Kffor both the impure solutions of Lauric acid should be the same. This is because it is simply the solvent, Lauric Acid, which holds the value of Kf. However, when averaging the two Kf’s out, I got a final Kfof 3.9C/m that is very close to the actual Kfof Lauric Acid.
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