31. The cryoscopic constant (Kf) of water is 1.86°C·kg/mol. If 0.5 mol of NaCl is dissolved in 1 kg of water, what is the freezing point depression of the solution? (Assume complete dissociation of NaCl, i=2)
a) 1.86°C
b) 2.79°C
c) 3.72°C
d) 5.58°C
Answer: c) 3.72°C
Explanation:
Freezing point depression is calculated using ΔTf=i×Kf×m
32. Which of the following represents an ideal solution?
a) Benzene and methanol
b) Acetone and chloroform
c) Benzene and toluene
d) Ethanol and water
Answer: c) Benzene and toluene
Explanation:
Ideal solutions follow Raoult’s law at all concentrations. Benzene and toluene have similar structures and intermolecular forces, making them behave ideally when mixed. Other combinations like ethanol and water or acetone and chloroform show deviations due to hydrogen bonding or strong interactions.
33. Which colligative property is used to determine the molar mass of biomolecules like proteins?
a) Boiling point elevation
b) Freezing point depression
c) Relative lowering of vapor pressure
d) Osmotic pressure
Answer: d) Osmotic pressure
Explanation:
Osmotic pressure is commonly used to determine the molar mass of large biomolecules like proteins, as small changes in osmotic pressure are easier to measure than other colligative properties for such large molecules.
34. In an aqueous solution, which of the following solutes will have the greatest impact on the colligative properties of the solution?
a) 1 M glucose
b) 1 M NaCl
c) 1 M urea
d) 1 M K₃[Fe(CN)₆]
Answer: d) 1 M K₃[Fe(CN)₆]
Explanation:
The more particles a solute dissociates into, the greater its impact on colligative properties. K₃[Fe(CN)₆] dissociates into 4 ions (3 K⁺ and 1 Fe(CN)₆³⁻), which affects colligative properties more significantly than NaCl (2 ions), glucose, and urea (no dissociation).
35. What is the van’t Hoff factor (i) for aluminum chloride (AlCl₃) in an aqueous solution?
a) 1
b) 2
c) 3
d) 4
Answer: d) 4
Explanation:
Aluminum chloride (AlCl₃) dissociates completely in water into one Al³⁺ ion and three Cl⁻ ions, giving a total of 4 particles. Therefore, the van’t Hoff factor for AlCl₃ is 4.
36. Which of the following explains why a non-volatile solute lowers the vapor pressure of a solvent?
a) The solute occupies the surface area, reducing the number of solvent molecules that can escape into vapor
b) The solute decreases the temperature of the solvent
c) The solute increases the molecular interactions between solvent particles
d) The solute increases the vapor pressure of the solvent
Answer: a) The solute occupies the surface area, reducing the number of solvent molecules that can escape into vapor
Explanation:
When a non-volatile solute is added to a solvent, it occupies some of the surface area, reducing the number of solvent molecules that can escape into the vapor phase, which lowers the vapor pressure of the solvent.
37. What is the effect of adding a non-volatile solute to a solvent on the boiling point of the solution?
a) It lowers the boiling point
b) It raises the boiling point
c) It has no effect on the boiling point
d) It lowers the vapor pressure but not the boiling point
Answer: b) It raises the boiling point
Explanation:
Adding a non-volatile solute to a solvent raises the boiling point of the solution. This phenomenon, known as boiling point elevation, occurs because the solute reduces the vapor pressure of the solvent, requiring a higher temperature for the vapor pressure to reach atmospheric pressure.
38. Which property remains unchanged when a solute is added to a solvent?
a) Boiling point
b) Freezing point
c) Vapor pressure
d) Molar mass of the solvent
Answer: d) Molar mass of the solvent
Explanation:
Adding a solute to a solvent changes properties like boiling point, freezing point, and vapor pressure (colligative properties), but the molar mass of the solvent remains unchanged as it is an intrinsic property of the solvent.
39. If the boiling point of a solution is 101°C and the solvent is water, what is the approximate molality of the solution if the ebullioscopic constant (Kb) of water is 0.52°C/m?
a) 1 m
b) 2 m
c) 0.5 m
d) 0.96 m
Answer: d) 0.96 m
40. For a solution exhibiting positive deviation from Raoult’s law, which of the following statements is correct?
a) The vapor pressure of the solution is higher than expected
b) The vapor pressure of the solution is lower than expected
c) The solution exhibits a freezing point lower than expected
d) The solute and solvent exhibit strong attractive forces
Answer: a) The vapor pressure of the solution is higher than expected
Explanation:
Positive deviation from Raoult’s law occurs when the vapor pressure of the solution is higher than expected because the interactions between solute and solvent molecules are weaker than those between the solvent molecules in the pure state.
41. Which of the following factors can affect osmotic pressure?
a) Temperature
b) Solute concentration
c) Nature of the solute particles
d) Both a and b
Answer: d) Both a and b
Explanation:
Osmotic pressure depends on both temperature and the concentration of solute particles. According to the van’t Hoff equation (Π=iMRT), osmotic pressure increases with an increase in solute concentration or temperature.
42. The cryoscopic constant (KfKf) of a solvent is:
a) The freezing point of the solvent
b) The increase in boiling point per molality of the solute
c) The depression in freezing point per molality of the solute
d) The vapor pressure lowering per mole fraction of solute
Answer: c) The depression in freezing point per molality of the solute
Explanation:
The cryoscopic constant (KfKf) represents the freezing point depression per molality of solute added to the solvent. It is specific to each solvent and is used to calculate the change in freezing point when a solute is dissolved.
43. The molar mass of a solute can be determined from which of the following properties?
a) Boiling point of the solution
b) Freezing point of the solution
c) Osmotic pressure of the solution
d) All of the above
Answer: d) All of the above
Explanation:
The molar mass of a solute can be determined from colligative properties such as boiling point elevation, freezing point depression, or osmotic pressure, as these depend on the number of solute particles in the solution.
44. What is the van’t Hoff factor (ii) for sucrose in an aqueous solution?
a) 1
b) 2
c) 3
d) 4
Answer: a) 1
Explanation:
Sucrose is a non-electrolyte and does not dissociate into ions in solution. Therefore, the van’t Hoff factor (ii) for sucrose is 1.
45. In an aqueous solution, which of the following will produce the lowest freezing point?
a) 0.1 M NaCl
b) 0.1 M CaCl₂
c) 0.1 M glucose
d) 0.1 M urea
Answer: b) 0.1 M CaCl₂
Explanation:
Freezing point depression depends on the number of solute particles. CaCl₂ dissociates into 3 ions (1 Ca²⁺ and 2 Cl⁻), producing more particles than NaCl (2 ions), glucose, or urea (no dissociation). Hence, 0.1 M CaCl₂ will have the greatest effect on freezing point depression.
46. Which of the following solutions will have the highest vapor pressure?
a) 1 M NaCl
b) 1 M glucose
c) 1 M K₂SO₄
d) 1 M CaCl₂
Answer: b) 1 M glucose
Explanation:
Vapor pressure lowering is a colligative property and depends on the number of solute particles. Glucose is a non-electrolyte and does not dissociate, so it will have the fewest solute particles in solution, resulting in the highest vapor pressure compared to the dissociating electrolytes (NaCl, K₂SO₄, and CaCl₂).
47. Which of the following factors increases the magnitude of colligative properties?
a) Lower solute concentration
b) Non-electrolyte solutes
c) Electrolytes that dissociate completely
d) Decrease in temperature
Answer: c) Electrolytes that dissociate completely
Explanation:
Colligative properties are directly related to the number of particles in solution. Electrolytes that dissociate completely produce more ions, increasing the magnitude of colligative properties like boiling point elevation, freezing point depression, and osmotic pressure.
48. Which of the following is a colligative property?
a) Surface tension
b) Viscosity
c) Freezing point depression
d) Density
Answer: c) Freezing point depression
Explanation:
Colligative properties are those that depend on the number of solute particles but not on their identity. Freezing point depression is a classic example of a colligative property.
50. Which of the following will exhibit negative deviation from Raoult’s law?
a) Benzene and toluene
b) Ethanol and water
c) Acetone and chloroform
d) Methanol and hexane
Answer: c) Acetone and chloroform
Explanation:
Negative deviation from Raoult’s law occurs when the intermolecular forces between the solute and solvent are stronger than those in the pure components. Acetone and chloroform form hydrogen bonds, leading to negative deviation.
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