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Molarity is arguably the most intuitive descriptor for the "number of particles". But freezing point depression dependence on solute concentration is commonly described in textbooks as a function of molality. The same textbooks make it seem as if freezing point depression formula was determined empirically, in which case, it should take take only a few corrective factors to change it into a dependence on molarity. (In fact, I wouldn't be surprised to learn that some correction factors should be used to make the currently formula more accurate. That is often the case with empirical formulae.)

So is it really the case that using molarity would make the formula dependent on solute nature, or was molality used for convenience?

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    $\begingroup$ Molality is the best value to use because it does not change with the density of the solvent, which is a significant factor when discussing freezing/melting points. $\endgroup$ – Dan Burden Aug 2 '16 at 0:21
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    $\begingroup$ Molarity depends on the volume so it also depends on other solutes while molality does not. $\endgroup$ – SteffX Aug 2 '16 at 1:05
  • $\begingroup$ @SteffX You are suggesting that it is convenience. I wish I'd see wither some theory-based deduction, or the experimental data showing molarity could not be used just as well. $\endgroup$ – Nick Alexander Aug 2 '16 at 23:44

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