Question

How does increasing the concentration of a nonvolatile solute in water affect the following properties: (a) vapor pressure, (b) freezing point, (c) boiling point; (d) osmotic pressure?

Short answer

Increasing the concentration of a nonvolatile solute in water affects the properties as follows: (a) Vapor pressure decreases because fewer water molecules can escape into the vapor phase; (b) Freezing point decreases due to solute particles disrupting ice formation; (c) Boiling point increases because more energy is required for water molecules to escape; (d) Osmotic pressure increases as solvent molecules flow toward higher solute concentration to achieve equilibrium.

Step-by-step solution

(a) Vapor Pressure

A nonvolatile solute does not have a considerable vapor pressure at any given temperature. When it is dissolved in water, the solute particles take up some of the water's surface area, which is initially occupied by water molecules. As a result, the number of water molecules that can escape into the vapor phase decreases, lowering the overall vapor pressure of the solution compared to that of pure water. As the concentration of the nonvolatile solute increases, the vapor pressure decreases even further.

(b) Freezing Point

Adding a nonvolatile solute to water lowers the freezing point of the solution, a phenomenon known as freezing point depression. The solute particles disrupt the formation of the ordered structure of ice, making it harder for water to freeze. Consequently, a greater amount of energy has to be removed from the solution for freezing to occur. As the concentration of the solute increases, the freezing point of the solution decreases further.

(c) Boiling Point

The boiling point of a solution containing a nonvolatile solute in water is higher than that of pure water, which is called boiling point elevation. This is because the solute particles interfere with the escape of water molecules into the vapor phase, requiring more energy (in the form of increased temperature) for the same amount of water molecules to escape. As the concentration of the solute increases, the boiling point of the solution increases.

(d) Osmotic Pressure

Osmotic pressure is the pressure required to prevent the flow of solvent molecules across a semipermeable membrane (which allows the solvent but not the solute to pass through) from an area of lower solute concentration to an area of higher solute concentration. When the concentration of a nonvolatile solute in water increases, the osmotic pressure also increases. This is because the solvent molecules are more likely to flow towards the area of higher solute concentration to achieve equilibrium.