general chemistry - using molality in determining termperature change

[Monkeymaniac]

While reading the stoichiometry section in TBR, I found a phrase that wasn't entirely clear. It states,

Molality is the concentration of a fluid solution defined as the moles of a solute per kilogram of solvent. The molality of a solution does not change with temperature, so it is often used to determine a change in the solution's temperature when the change depends on concentration. Notable examples of this include boiling-point elevation and freezing-point depression.

How exactly is the quantity that doesn't change with temperature used in determining the temperature change? Also does the word concentration used in the later sentence refer to the concentration mentioned in the first sentence, or is it talking about the conentration in a traditinoal sense (#moles/volume). I know I am missing something here. Does anyone have an idea as to what the book was trying to allude?

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[bakanoisha]

It's been a while since I did this crap, but I think they're basically just trying to say that because molality doesn't change in a solution it makes things a whole lot easier to deal with. dT = iKM where M=molality, K=constant. If 'M' wasn't constant, this would be a little more complicated to solve.

Oh, and remember, this dT refers to the change in the temperature at which the solvent WILL freeze or boil as a result of adding some solute to the solvent. Adding the solute to the solvent to create a solution may, or may not, actually change the temperature some appreciable amount. In these MCAT instances, typically we assume that the T before addition of solute is the same as T after.

 

[bakanoisha]

Oh, and one last thing... I think you need to realize what's "physically" happening here. Adding, for example, Na+Cl- to H2O results in ionized Na+ and Cl- in the polar water solution, which will orient itself around the ions in an relatively orderly fashion due to electrostatic interactions. When this happens the ions make creating a crystallized structure more difficult b/c thereby decreasing the temperature at which the water can freeze (or crystalize). At the same time they essentially anchor down water molecules from escaping into the gas phase. So, when you boil water that has added Na+Cl- the more Na+Cl- there is the more the water is "anchored down" (hindered from leaving the liquid phase to the gas phase). This means more energy is required to get the water to enter the gas phase, and thus a higher temperature of boiling results.