Which one have more vapour pressure 0.5m aquous NaCl solution or 0.5m Urea soluion at the same temperature and pressure - explain?

To determine which solution has a higher vapor pressure between a 0.5m aqueous NaCl solution and a 0.5m urea solution at the same temperature and pressure, we need to consider the concept of vapor pressure lowering, which is influenced by the number of solute particles in a solution.

Understanding Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase. When a non-volatile solute is added to a solvent, the vapor pressure of the solvent decreases. This phenomenon occurs because the solute particles occupy space at the surface of the liquid, reducing the number of solvent molecules that can escape into the vapor phase.

Colligative Properties

The decrease in vapor pressure is a colligative property, which means it depends on the number of solute particles in a solution rather than the identity of the solute. In this case, we have two different solutes: sodium chloride (NaCl) and urea.

Breaking Down the Solutions

• NaCl Solution: Sodium chloride is an ionic compound that dissociates into two ions when dissolved in water: Na⁺ and Cl^-. Therefore, a 0.5m NaCl solution produces 0.5 moles of Na⁺ and 0.5 moles of Cl^-, resulting in a total of 1.0 mole of particles per liter of solution.
• Urea Solution: Urea, on the other hand, is a non-ionic compound that does not dissociate in solution. A 0.5m urea solution simply contains 0.5 moles of urea molecules per liter of solution.

Comparing the Two Solutions

Now, let's compare the number of solute particles in each solution:

• The 0.5m NaCl solution has 1.0 mole of particles (0.5 moles of Na⁺ and 0.5 moles of Cl^-).
• The 0.5m urea solution has only 0.5 moles of particles (0.5 moles of urea).

Conclusion on Vapor Pressure

Since the vapor pressure lowering is directly related to the number of solute particles, the 0.5m NaCl solution, with its 1.0 mole of particles, will exhibit a greater reduction in vapor pressure compared to the 0.5m urea solution, which has only 0.5 moles of particles. Consequently, the vapor pressure of the urea solution will be higher than that of the NaCl solution at the same temperature and pressure.

In summary, the 0.5m urea solution has a higher vapor pressure than the 0.5m aqueous NaCl solution due to the greater number of solute particles present in the NaCl solution, which leads to a more significant lowering of vapor pressure.