Molarity and molality are both used to express the concentration of a solution. The amount of solute is both expressed in moles.
Their major difference is for Molarity, it is expressed as the moles per Liter/s of solution while for Molality its moles per Kilogram/s of solvent.
Molarity (M) = moles of solute / Liter(s) of solution
Molality (m) = moles of solute / Kilogram(s) of solvent
Take not of the use of symbols (capital letter M for molarity and small letter m for molality)
To avoid confusion, always remember that:
Volume of solution = volume of solute + Volume of solvent
Kilogram of Solution = Kilogram of solute + Kilogram of solvent
Tip: you can use the density of the solution in working with this kind of problem.
Molality vs. Molarity:
These two words sound similar but they are not synonyms, even though both of them are used for representing solution concentration. By definition, molarity is the number of moles of solute dissolved per liter of solution. We use capital letter “M” to represent molarity and its formula is M= (# mol SOLUTE)/ (Liters of SOLUTION).
Molality is then the number of moles of solute per kilogram of the SOLVENT, NOT solution! We use lower case letter “m” to represent molality and its formula can be represented as: m= (# mole SOLUTE) / (Kilograms of SOLVENT).
Most of the time scientists use either molarity or molality to represent solution concentration, but MOLALITY is preferred when the temperature of the solution varies. That is because MOLALITY does not depend on temperature, (Neither number of moles of solute nor mass of solvent will be affected by changes of temperature.) while MOLARITY changes as temperature changes. (Volume of solution in the formula changes as temperature changes, and that is why.)