### Video Transcript

The given graph shows vapor pressure against temperature for three solutions of NaCl and pure water. Which solution has the highest concentration?

The provided graph represents the vapor pressure in kilopascals of three aqueous solutions of sodium chloride and pure water as a function of temperature in degrees Celsius. To solve this problem, we need to determine which of the three solutions depicted in the graph has the highest concentration of sodium chloride.

Vapor pressure is defined as the pressure exerted by a vapor above its liquid when they are at dynamic equilibrium. In a closed system at constant temperature and pressure, dynamic equilibrium is reached when the rates of condensation and evaporation are equal. At this point, the vapor pressure will remain constant. All of the curves on the graph show a positive upward trend, which indicates that vapor pressure increases with temperature. This is because as we increase the temperature of a liquid, the rate of evaporation also increases, producing more vapor.

We also notice something even more interesting on this graph. All three of the sodium chloride solutions have lower vapor pressure than pure water at all temperatures. For example, at 90 degrees Celsius, pure water has the highest vapor pressure and solution C has the lowest vapor pressure.

To help us better understand this observation, we need to discuss colligative properties of solutions. Colligative properties are properties of a solution that depend on the number of solute particles in the solution, not the identity of the solute. In the solutions in this problem, the solute is sodium chloride and the solvent is water. When solid sodium chloride is dissolved in water, an aqueous solution of sodium chloride forms. The dissolved solute particles, which in this case are sodium and chloride ions, interfere with the process of evaporation of the water molecules.

If solvent molecules cannot evaporate as easily, then less vapor molecules will be present above the solution. In other words, adding a solute to water lowers the vapor pressure. Let’s say we measure the vapor pressure of water and all three solutions at 90 degrees Celsius and all four samples have the same volume. Dissolving some sodium chloride into pure water produces solution A, which has a lower vapor pressure than pure water. Dissolving more sodium chloride into solution A produces solution B, which has an even lower vapor pressure. Finally, dissolving even more sodium chloride into solution B produces solution C, which has the lowest vapor pressure of all three solutions. Solution C contains the most dissolved solute particles. Therefore, it is the most concentrated solution of sodium chloride.

On the graph, the curve with the lowest vapor pressure at any temperature is the curve representing solution C. Therefore, solution C has the highest concentration of sodium chloride.