What are some common mistakes students make with molality?
Probably the most common mistake is confusing it with molarity.
The names differ by only one letter: molaLity - molaRity. Both express concentration. Their definitions seem almost as close as their names.
Molality is the moles of solute divided by the kilograms of solvent .
Molarity is the moles of solute divided by liters of solution .
Note the different "denominators." This is especially confusing with water because 1 kg = 1 L. But when you make a solution the volume of the solution will not be exactly the same as the solvent you begin with. The added solute will change the volume. So 1 kg of water (solvent) will usually make slightly more than 1 L of solution.
Be careful with definitions and units. ALWAYS keep track of units. If you pay attention to them units will become your best friends in chemistry. Ignore them at great peril.
Another common mistake occurs when working with colligative properties. Boiling point and freezing point are affected by total molality. Students often forget to take into account how ionic compounds break into more than one particle. That makes the total molality used for the calculation larger than the molality of the compound. For example, if a NaCl solution is 0.01 m, the total molality for calculating b.p. elevation or f.p. depression would be 0.02 m,
I hope this helps.