When a solution and the pure solvent are separated by a semipermeable membrane, the solution exerts a pressure on the membrane called osmotic pressure. The osmotic pressure increases with increase in number of particles, ions or molecules, in the solution. If 10 millimoles of each of the sulphate salts of sodium, magnesium and aluminium are dissolved in 1.0 litre of water in three different beakers labelled as P, Q and R respectively, the osmotic pressure follows the order