# Colligative properties problems and solutions

1. What are colligative properties? Solution: Colligative properties are properties of solutions that depend on the ratio of the number of solute particles to the number of solvent particles, and not on the nature of the solute or solvent.

2. Name four colligative properties. Solution: The four main colligative properties are boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.
3. How does a nonvolatile solute affect the boiling point of a solvent? Solution: The presence of a nonvolatile solute elevates the boiling point of a solvent. This is because the solute particles interfere with the vaporization process, requiring more heat (higher temperature) to change the liquid to a gas.
4. How does a solute affect the freezing point of a solvent? Solution: The presence of a solute lowers the freezing point of a solvent. This is because solute particles interfere with the orderly arrangement of solvent particles required to form a solid, requiring a lower temperature to freeze.
5. What is the van’t Hoff factor and how does it relate to colligative properties? Solution: The van’t Hoff factor (i) is the ratio of the actual number of particles in solution after dissociation to the number of formula units initially dissolved in the solution. It is used to correct colligative property calculations for solutions of ionic compounds, which dissociate into more than one particle.
6. How does the presence of a solute affect the vapor pressure of a solvent? Solution: The presence of a solute decreases the vapor pressure of a solvent. This is because the solute particles reduce the number of solvent particles that can escape to the vapor phase, reducing the vapor pressure.
7. Why does adding salt to ice lower its melting point? Solution: The addition of salt to ice lowers its melting point, a phenomenon known as freezing point depression. The salt particles interfere with the ability of the water molecules to arrange themselves into a solid structure, so a lower temperature is required for freezing.
8. What is osmotic pressure? Solution: Osmotic pressure is the pressure that needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.
9. How does the molar mass of a solute affect the boiling point elevation and freezing point depression of a solvent? Solution: The molar mass of a solute directly affects the boiling point elevation and freezing point depression of a solvent. A solute with a larger molar mass will produce fewer particles in solution for the same mass, resulting in a smaller change in boiling point and freezing point.
10. Why does a 1 molal solution of glucose in water have a lower freezing point depression than a 1 molal solution of NaCl in water? Solution: A 1 molal solution of NaCl produces more particles in solution than a 1 molal solution of glucose because NaCl dissociates into two ions (Na⁺ and Cl⁻) in water, while glucose does not dissociate. Since freezing point depression depends on the number of particles, the NaCl solution has a greater freezing point depression.
11. How does the presence of a solute affect the colligative properties of a solution? Solution: The presence of a solute changes the colligative properties of a solution. It lowers the freezing point and increases the boiling point of the solution. It also decreases the vapor pressure and can increase the osmotic pressure of the solution.
12. Why does a solution boil at a higher temperature than the pure solvent? Solution: A solution boils at a higher temperature than the pure solvent because the solute particles interfere with the vaporization process, requiring a higher temperature to overcome the intermolecular forces keeping the solvent in the liquid phase.
13. How does the concentration of a solution affect its colligative properties? Solution: The concentration of a solution directly affects its colligative properties. The higher the concentration of the solution (i.e., the more solute particles per solvent particles), the greater the changes in boiling point, freezing point, vapor pressure, and osmotic pressure.
14. Why does the vapor pressure of a solution decrease when a nonvolatile solute is added? Solution: The vapor pressure of a solution decreases when a nonvolatile solute is added because the solute particles reduce the number of solvent particles that can escape to the vapor phase.
15. How is osmosis related to colligative properties? Solution: Osmosis is a colligative property. It involves the movement of solvent molecules from a region of lower solute concentration to a region of higher solute concentration. The pressure that needs to be applied to stop this flow, the osmotic pressure, depends on the number of solute particles, not their nature.
16. How can colligative properties be used to determine molar mass? Solution: Colligative properties can be used to determine molar mass by measuring the change in a property (e.g., freezing point depression) for a known amount of solute and solvent, and then using the colligative property equation to solve for molar mass.
17. What happens to the colligative properties of a solution if the solute dissociates into more than one particle? Solution: If the solute dissociates into more than one particle, the effect on the colligative properties is multiplied by the number of particles (the van’t Hoff factor). For example, NaCl dissociates into two particles, so it would double the colligative effect compared to a non-dissociating solute of the same concentration.
18. Why is antifreeze added to the radiator fluid in a car? Solution: Antifreeze is added to lower the freezing point of the radiator fluid. This is a practical application of the colligative property known as freezing point depression. It helps prevent the fluid from freezing in cold temperatures.
19. How does the presence of a solute affect the phase diagram of a solvent? Solution: The presence of a solute generally lowers the freezing point and raises the boiling point of the solvent, changing the phase diagram. The boundary lines between the phases shift, reflecting the changed conditions for phase transitions.
20. Why does sea water have a higher boiling point and a lower freezing point than fresh water? Solution: Sea water has dissolved salts, making it a solution, while fresh water is a pure substance. The presence of solute particles in sea water elevates the boiling point and depresses the freezing point compared to fresh water due to the colligative properties of solutions.