Two solutions which have the same osmotic pressure at a given temperature are said to be isotonic in nature. The osmotic pressure of a given solution is given by the formula
π = CRT = n/VRT
Where π is the osmotic pressure of the solution, C is the concentration, the moles of solute per volume of solvent and T is the temperature, with R being universal gas constant. Osmotic pressure is a colligative property, hence for two solutions to be isotonic, the nature of the solute does not matter, i.e. the solute in the two solutions do not have to be the same. Even the density, which does not have a place in the equation of osmotic pressure does not need to be the same. The isotonic solutions at a given temperature need to have the same volume and same molar concentration, hence they will also have the equal elevation in boiling point and depression in freezing point, two other colligative properties.
The correct answers are (iii) and (iv).
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