How high can molality be?

1 Answer
Feb 2, 2014

The maximum molality is 52.8 m.

In practical terms, the highest molarity for aqueous solutions is that of the solvent itself, water.

One litre of pure water has a mass of 1 kg. Since the molar mass of water is 18.02 g/mol, the molality of pure water is 55.49 mol/kg or 55.49 m.

In a solution, the solvent must be the major component. That means we would need to have less than 1000 g of solute in 1 kg of water. Not many substances are this soluble. The only compound that comes close is NH₃, which has a solubility of 900 g/kg water.

If we mixed 900 g of NH₃ (molar mass = 17.03 g/mol) with 1000 g of water, we would have a 52.8 m solution of NH₃. This is therefore the maximum practical molality for aqueous solutions.