What is the osmolarity of a 0.7 m KNO_3 solution, assuming complete dissociation?

• A. 0.35 osmol/L
• B. 0.7 osmol/L
• C. 1.4 osmol/L
• D. 2.1 osmol/L

Answer: (C)

Verified step by step guidance

1. Identify the meaning of molality (m): it is the number of moles of solute per kilogram of solvent. Here, 0.7 m KNO_3 means 0.7 moles of KNO_3 per 1 kg of solvent.
2. Recognize that osmolarity depends on the total concentration of dissolved particles (osmoles) in solution. Since KNO_3 dissociates completely into K^+ and NO_3^- ions, each mole of KNO_3 produces 2 osmoles.
3. Calculate the total osmoles contributed by the dissociation: multiply the molality by the number of particles formed upon dissociation. For KNO_3, this is 0.7 m × 2 = 1.4 osmol/kg solvent.
4. Convert molality to molarity (osmoles per liter) if necessary. Assuming the density of the solution is close to that of water, molality and molarity values can be approximated as equal for dilute solutions, so osmolarity ≈ 1.4 osmol/L.
5. Conclude that the osmolarity of the 0.7 m KNO_3 solution, assuming complete dissociation, is approximately 1.4 osmol/L.