The molality of 85-percent phosphoric acid is 57.6 m, or mol/kg. This value can be calculated using the mass density and molarity of the 85-percent phosphoric acid solution, which are 1.70 g / mL (1.70 kg / L) and 14.7 mol / L, respectively.
Molality is defined as the number of mols of solute divided by the mass of the solvent given in kilograms. In an 85-percent phosphoric acid solution, the mass of the solvent is 15 percent of the total mass. This means that the mass of water in a liter of 85-percent phosphoric acid solution would be 0.255 kg. The molarity of 85-percent phosphoric acid is 14.7 mol / L, so in one liter of solution, there would be 14.7 mols. Thus, the molality in mols / kg is 14.7 mol / 0.255 kg or 57.6 mol / kg.