Rank the following solutions from lowest to highest vapor pressure. Rank from lowest to highest. To rank items as equivalent, overlap them. 1. 20 g. of glucose C6H12O6 in 100ml h2o 2. 10 g. of potassium acetate KC2H3O2 in 100ml h2o 3. 20 g. of sucrose C12H22O11 in 100ml h2o
The Correct Answer and Explanation is:
Correct Ranking (from lowest to highest vapor pressure):
3 = 1 < 2
Explanation:
The vapor pressure of a solution depends on the number of solute particles in the solution, not on their chemical identity. This is a colligative property, which means it depends on the quantity of solute particles rather than their type. The key factors affecting vapor pressure in solutions are:
- Number of particles (ions or molecules) the solute yields when dissolved
- Molar concentration of the solute
Let’s analyze each solution:
1. 20 g of glucose (C₆H₁₂O₆) in 100 mL water
- Glucose is a non-electrolyte, meaning it does not dissociate into ions in water.
- Molar mass = ~180 g/mol
- Moles = 20 g / 180 g/mol ≈ 0.111 mol
- Since it does not ionize, it contributes 0.111 mol of solute particles.
2. 10 g of potassium acetate (KC₂H₃O₂) in 100 mL water
- This is a strong electrolyte, and it dissociates completely into K⁺ and C₂H₃O₂⁻.
- Molar mass = ~98 g/mol
- Moles = 10 g / 98 g/mol ≈ 0.102 mol
- It dissociates into 2 ions, so particle concentration = 0.102 mol × 2 = 0.204 mol particles
- This is more particles than the glucose or sucrose solutions, so it will have a higher vapor pressure (lower reduction in vapor pressure).
3. 20 g of sucrose (C₁₂H₂₂O₁₁) in 100 mL water
- Sucrose is also a non-electrolyte, like glucose.
- Molar mass = ~342 g/mol
- Moles = 20 g / 342 g/mol ≈ 0.058 mol
- Does not dissociate, so particle concentration = 0.058 mol
- Fewer particles than glucose or potassium acetate → lowest vapor pressure.
Final Ranking (Lowest to Highest Vapor Pressure):
- Lowest: Sucrose (fewer particles)
- Glucose and sucrose are similar non-electrolytes, but glucose yields more moles → slightly higher vapor pressure than sucrose
- Highest: Potassium acetate (electrolyte, more dissociated particles)
Because glucose and sucrose are both non-electrolytes, and their particle concentrations are fairly close, we consider them effectively equal in vapor pressure for ranking purposes. Thus:
Correct Answer: 3 = 1