Molality

Molality(m) is the number of moles of solute per kilogram of solvent. Note the difference between the formula for molarity & molality. Both have moles of solute as the numerator. For molarity, the denominator is liters of solution, for molality it is kilograms of solvent. Also, the symbol for molarity is a capital M, for molality, it is a lower case m.

For example: If 38.0 grams of sodium chloride is dissolved in 880. grams of water, what is the molality of the solution?

You must first convert grams to moles by dividing the compound's molar mass.
(38.0 grams sodium chloride) / (58.5 grams/mole) = 0.650 moles sodium chloride

Next convert grams of solvent to kilograms of solvent.
880. / 1000. = 0.880 kilograms

Now, plug it in the formula and solve.
Molality = 0.650 moles / 0.880 liter = 0.739 Molal

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