Molality: Comparing solute to solvent

Molality is another concentration term that involves moles of solute. It isn’t used very much, but you may happen to run across it. Molality (m) is defined as the moles of solute per kilogram of solvent. It’s one of the few concentration units that doesn’t use the total solution’s weight or volume. Mathematically, it looks like this:

Suppose, for example, you want to dissolve 15.0 grams of NaCl in 50.0 grams of water. You can calculate the molality like this (you must convert the 50.0 grams to kilograms before you use it in the equation):