When chemicals are dissolved in water they interact with the water molecules and change the physical properties of the resulting solution. The last statement is easily proved by dissolving some sugar in exactly one liter of water. The volume of the final solution will be more than one liter and, thus, the number of water molecules in a liter of the solution will be lower than that in a liter of pure water.
All molecules have a molecular weight which is determined by their composite atoms.
One GMW of such a solute dissolved in one liter of pure water is called a one molal solution and it has an osmotic pressure of one osmole or 1000 milliosmoles. Human blood plasma has an osmotic pressure of about 290 mOsm and water from the open ocean has an osmotic pressure of about 1010 mOsm. This indicates that there is much more salt in sea water than in human blood plasma and also that there is less pure water in a liter of sea water than in a liter of plasma. In considering this it is well to remember that salt added to water lowers the water concentration of the resulting solution. The exact opposite happens when we immerse ourselves in sea water at the beach and our skin, particularly on our hands and faces, shrinks by osmosis and feels tight.