Osmotic pressure is a colligative property, meaning that the osmotic pressure depends on the molar concentration of the solute but not on its identity.
From the en.wikipedia.org
Another method is the less well-known metabisulfite method, in which metabisulfite is placed at the bottom of a beaker, and 12.6 molar concentration hydrochloric acid is added.
From the en.wikipedia.org
If specific molar values are given, then the amount of substance of a given sample may be determined by measuring volume, mass or concentration.