How do you calculate molality of a solution?

1 Answer
Jan 25, 2014

Molality is a measurement of the concentration of a solution by comparing the moles of the solute with the kilograms of the solvent the solute is dissolved in.

If a solution of salt water contains 29 grams of sodium chloride (NaCl) and that salt is dissolved in 1000 grams of water, the molarity can be determined by converting the grams of sodium chloride to moles and dividing that by the mass of the water converted to kilograms.

Since the molar mass (gram formula mass of sodium chloride is 58 grams per mole ( Na = 23 g and Cl = 35 g , 23 + 35 = 58 g/mol)
the mole value of the NaCl is 0.5 moles (29 g / 58 g/mol = 0.5 moles).

The mass of water is 1000 grams which is converted to 1.0 kg.

Molality = moles of solute / kg of solvent.

Molality = 0.5 moles / 1.0 kg = 0.5 molal