Osmosis happens when semipermeable materials/membranes allow small particles to pass through while larger particles are blocked. But it only happens when two solutions have different osmotic pressure. And osmotic pressure is equal to (n/v) * R*T (T represents temperature and the unit must be Kelvin) . Sometimes some questions do not give number of moles and volume, but give molarity instead. But since n/v equals molarity by definition of molarity, osmotic pressure is then also equal to M*R*T.  By the way, when osmosis happens, the direction is always from the side with more solvent to the side with less solvent, or we can say that’s from the side with lower concentration of solute to the side with higher concentration of solute.

We have discussed the situation when two solutions have different osmotic pressures. Then what about two solutions that have the same osmotic pressure? As what is previously stated, no osmotic phenomenon is going to happen. And these two solutions will be regarded as isotonic.

We can describe the relationship between two solutions that have different osmotic pressures this way: If a solution has a relatively lower osmotic pressure, then it is hypotonic with respect to the solution with higher concentration; the more concentrated solution is hypertonic with respect to the solution with lower concentration.