OK, everybody concentrate...

Email question...

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I am studying for the exams, and I just had a random question. Why are there Molarity and Molality? Aren't they almost always going to be the same number since 1 g H2O = 1 mL H2O? I guess I'm just curious of when you would choose one measurement over the other.

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In practice, yes, they are almost always the same (to our ability to measure them...), but the reasons we need them both are:

1. Molality is mols of solute per kilogram of solvent. Since the mass of a substance does not depend upon its temperature, molality is a concentration unit that is independent of temperature.

2. The density of pure water at (somewhere in the neighborhood of) 17degC is exactly 1g/mL, with many zeros of sig figs after the decimal point. For most dilute aqueous solutions at moderate temperatures, it's pretty safe to assume that the density of the solution is pretty close to 1g/mL, but there aren't nearly as many sig fig zeros after the decimal point. If you're working with a solution that's not dilute or not relatively close to that 15-20degC temperature, the density will start to vary significantly. That means the molarity (M) will change, but the molality (m) should not, therefore the conversion between these two units will not be 1:1. For an example you can try at home (or Kise...), take a little bit of water and add sugar or table salt to it until you get a very concentrated solution. Carefully (and slowly) pour that solution down the inside wall of a glass of "pure" water... the solution is more dense than the pure water and will sink to the bottom without mixing (if you're very careful).

3. What if your solvent is not water? In your freezing point depression experiment, your solvent was cyclohexane, so there was absolutely NOT a 1:1 relationship between the M and m of the solutions you were making.

Again, in many "real" situations the difference between M and m in an aqueous solution you're using will be so small that it's ignored. Similarly for other concentration unit conversions, we tend to simplify them in practice because the solutions we are most often working with in a lab (and especially a biology or biochemistry lab) are probably aqueous and probably relatively dilute. As long as you understand the assumptions you're using to simplify your in-lab calculations, you should be able to adjust to different situations or solvents...