1) The freezing point of a solution is depressed compared to that of a pure solvent. The difference in freezing point depends on the identity of the solvent and on the quantity (but not the identity) of dissolved solute. The quantitative relationship is:(To download these data in a spreadsheet file, click here.)
Substance molecular weight depression of freezing point, °C methyl iodide 142 0.335 chloroform 119.5 0.428 carbon tetrachloride 154 0.333 carbon disulfide 76 0.654 ethyl iodide 156 0.331 ethyl bromide 109 0.461 hexane 86 0.597 ethylene chloride 99 0.491 turpentine (α-pinene) 136 0.366 nibrobenzene 123 0.390 naphthalene 128 0.391 anthracene 178 0.287 methyl nitrate 77 0.640 dimethyl oxalate 118 0.417 methyl salicylate 152 0.339 diethyl ether 74 0.671 diethyl sulfide 90 0.576 ethyl nitrile 55 0.938 ethyl formate 74 0.666 ethyl valerate 130 0.384 allyl thiocyanate 99 0.519 nitroglycerine 227 0.220 trigylceride of butyric acid 302 0.161 trigylceride of oleic acid 884 0.056 acetaldehyde 44 1.107 chloral 147.5 0.342 benzaldehyde 106 0.473 camphor 152 0.338 acetone 58 0.850 dibutyl ketone 142 0.359
ΔT = Kfm ,where ΔT is the freezing point of the pure solvent minus that of the solution, m is the molality of the solution (moles of solute per kg of solvent), and Kf the freezing-point depression constant or cryoscopic constant. For any three solutes from the table above, compute the molality of the solution (1.00 g solute in 100. g solvent) and determine Kf.
2) Data such as those in the table above established the law of freezing point depression. As Raoult wrote in paper in which he presented these data,
"One can say, so far, that in a multitude of cases, the depression of the freezing point of a solvent depends only on the ratio of the numbers of molecules of the dissolved substance and of solvent; it is independent of the nature, the number, the arrangement of the atoms which compose the dissolved molecules."Use a spreadsheet to compute the freezing point depression constant from the data for all of the solutes. How constant is the freezing-point depression constant? Compute the mean and standard deviation of the constant according to these data. Are there any data points more than two standard deviations from the mean? Would it be justified to discard such points?
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