What does it mean to say two solutions are isotonic?

Answer 1

Osmolarity of a solution depends on concentration of the osmotically active particles in a solution. if 2 solutions have equal concentrations of such solvents these solutions are said to be isotonic to each other. If one molecule of any solvent dissociates into 2 particles in a solution, like sodium chloride, then half of its gram molecular weight/L will have same osmotic effect as one gram molecular weight of glucose/L, which provides one particle/molecule in a solution.