Question:   What's the difference between molarity and molality?

Answer:

Molarity is defined as the number of moles of solute per liter of solution.  This means that if you have a 1 M solution of some compound, evaporating one liter will cause one mole of the solute to precipitate.

Molality is defined as the number of moles of solute per kilogram of solvent.  To make a 1 m solution, you'd take one mole of a substance and add it to 1 L of solvent.  As a result, the final volume of a 1 m solution will be somewhat more than 1 L.