Van' t Hoff Factor Abnormal Molar Masses - UNSOLVED PRACTICE SET
Chapter: Solutions | Topic: Van t Hoff Factor Abnormal Molar Masses
VAN' T HOFF FACTOR ABNORMAL MOLAR MASSES - UNSOLVED PRACTICE SET
Topic: Van t Hoff Factor Abnormal Molar Masses
Multiple Choice Questions
Q1. The van 't Hoff factor (i) is defined as:
- Normal molar mass / Observed molar mass
- Observed molar mass / Normal molar mass
- Number of moles after dissociation / Number of moles before dissociation
- Both (a) and (c) are correct
Q2. For a solute that undergoes complete dissociation into two ions (like NaCl), the value of i is:
- 0
- 1
- 2
- Between 1 and 2
Q3. For a solute that undergoes association (dimerisation) in solution, the value of i is:
- Greater than 1
- Equal to 1
- Less than 1
- Equal to 2
Q4. The degree of dissociation (α) is related to van't Hoff factor (i) for a compound yielding n ions by:
- α = (i - 1) / (n - 1)
- α = (i + 1) / (n + 1)
- α = i / n
- α = (n - i) / (n - 1)
Q5. Acetic acid in benzene shows an abnormal molar mass because it:
- Dissociates into ions
- Associates to form dimers through hydrogen bonding
- Decomposes into simpler molecules
- Undergoes polymerisation
Q6. KCl in water shows i slightly less than 2 because:
- It does not dissociate completely
- Some ions recombine (ion pairing)
- It undergoes hydrolysis
- Both (a) and (b)
Short Answer Questions
Q7. Define van't Hoff factor. For each of the following, predict the value of i (assuming complete dissociation/association):
(a) Glucose in water
(b) NaCl in water
(c) K₂SO₄ in water
(d) Benzoic acid in benzene (dimerises)
Q8. The observed molar mass of NaCl, determined by freezing point depression, is less than 58.5 g/mol. Explain why this happens.
Q9. Calculate the van't Hoff factor for a 0.1 M solution of acetic acid (CH₃COOH) if its degree of dissociation is 1.3%.
Q10. Why does the van't Hoff factor decrease with increasing concentration of an electrolyte solution?
Q11. Your teacher prepares two solutions — one of urea and one of NaCl — both containing the same mass (5.85 g) in 1 kg of water. Predict which solution will show greater freezing point depression and explain using the van't Hoff factor.
Q12. Benzoic acid in benzene shows an observed molar mass of approximately 244 g/mol, while its normal molar mass is 122 g/mol. Calculate the van't Hoff factor and explain the reason for this abnormal value.
Long Answer Questions
Q13. Discuss the van't Hoff factor and abnormal molar masses in detail:
(a) Definition of van't Hoff factor (i) and its significance
(b) For electrolytes: i > 1 due to dissociation — relationship between i and degree of dissociation (α)
(c) For association: i < 1 — relationship between i and degree of association (α)
(d) Modified expressions for all colligative properties including i
(e) Why observed molar mass differs from normal molar mass
(f) Examples: NaCl, K₂SO₄, CH₃COOH in water; benzoic acid, acetic acid in benzene
Q14. Explain the dissociation of electrolytes and its effect on colligative properties:
(a) Strong electrolytes — complete dissociation (NaCl, KCl, K₂SO₄)
(b) Weak electrolytes — partial dissociation (CH₃COOH, NH₄OH)
(c) Calculation of degree of dissociation (α) from i
(d) Effect of concentration on i — dilution increases dissociation
(e) Debye-Hückel theory and ion pairing — why i is slightly less than theoretical even for strong electrolytes
(f) Comparison of experimental and theoretical values of i for various electrolytes
Q15. Understanding abnormal molar masses has practical significance in Indian science and industry. Discuss:
(a) Why determining molar mass by colligative properties requires knowledge of van't Hoff factor for electrolytes
(b) How the degree of dissociation of weak acids is determined in Indian pharmaceutical quality control labs
(c) The role of ion pairing in concentrated saline solutions used in chemical industries
(d) Why benzoic acid dimerisation in non-polar solvents is important for extraction processes in Indian spice and essential oil industries
Numerical / Application-Based Problems
Q16. A 0.5 molal solution of KCl freezes at -1.86°C. (Kf for water = 1.86 K·kg/mol, freezing point of pure water = 0°C).
(a) Calculate the observed freezing point depression.
(b) Calculate the van't Hoff factor (i).
(c) Calculate the degree of dissociation (α) of KCl in this solution.
(d) Compare with the theoretical value of i = 2 and explain any discrepancy.
Q17. Acetic acid (CH₃COOH) in benzene associates to form dimers. A solution containing 1.2 g of acetic acid in 100 g of benzene freezes at 4.92°C. (Pure benzene freezes at 5.50°C, Kf = 5.12 K·kg/mol).
(a) Calculate the observed molar mass of acetic acid in benzene.
(b) Calculate the van't Hoff factor (i).
(c) Calculate the degree of association (α).
(d) Explain why acetic acid dimerises in benzene but not in water.
[Given: Normal molar mass of CH₃COOH = 60 g/mol]
Q18. A pharmaceutical company in Hyderabad needs to determine the purity of a sample of NaCl intended for IV saline preparation. They use freezing point depression measurements.
(a) A solution of 5.85 g of the sample in 1 kg of water freezes at -3.58°C. Calculate the van't Hoff factor.
(b) If pure NaCl should give i = 1.9 at this concentration, calculate the percentage purity of the sample.
(c) If the impurity is non-volatile and non-electrolytic, calculate its mass percentage in the sample.
(d) Explain why even small deviations in i matter for medical-grade saline and the health risks of impure IV solutions.
[Given: Kf for water = 1.86 K·kg/mol, molar mass of NaCl = 58.5 g/mol]