Two solutions are isotonic if they have same molar concentrations of the particles. (a) 0.15 M NaCl and 0.1 M Na2SO4 NaCl is an electrolyte which dissociates to give 2 ions, thus concentration of ions in the solution 0.30 M. Similarly for Na2SO4 (3 ions), the concentration of ions in the solution = 0.30 M. Hence, both are isotonic.