ACS CP Chemical Principles & Laboratory Techniques 2 — Questions and Answers
Question 1: Which colligative property is used to determine the molar mass of a solute via osmotic pressure measurement?
- Boiling point elevation
- Freezing point depression
- Osmometry (Correct answer)
- Vapor pressure lowering
Correct answer: Osmometry
Osmometry measures osmotic pressure (π = MRT), which can be used to calculate molar mass, especially for polymers.
Question 2: In a thermogravimetric analysis (TGA), a sample loses 18% mass at 100°C. This loss most likely corresponds to:
- Combustion of organic matter
- Loss of adsorbed water (Correct answer)
- Decomposition of carbonate
- Reduction of a metal oxide
Correct answer: Loss of adsorbed water
A mass loss at ~100°C is characteristic of desorption of physisorbed or loosely bound water from the sample.
Question 3: Which technique uses the principle that analytes with different polarities elute at different times through a stationary phase?
- Atomic absorption spectroscopy
- High-performance liquid chromatography (Correct answer)
- Potentiometric titration
- Karl Fischer titration
Correct answer: High-performance liquid chromatography
HPLC separates analytes based on differential interactions with the mobile and stationary phases according to polarity or other properties.
Question 4: A buffer solution is most effective when the pH is within what range relative to the pKa of the weak acid?
- Within ±0.5 pH units
- Within ±1.0 pH unit (Correct answer)
- Within ±2.0 pH units
- Within ±3.0 pH units
Correct answer: Within ±1.0 pH unit
Buffers are most effective within ±1 pH unit of the pKa, where the ratio of conjugate base to acid is between 0.1 and 10.
Question 5: What does the van't Hoff factor (i) equal for MgCl₂ when fully dissociated in solution?
- 1
- 2
- 3 (Correct answer)
- 4
Correct answer: 3
MgCl₂ dissociates into one Mg²⁺ and two Cl⁻ ions, giving a van't Hoff factor of 3.
Question 6: Which type of laboratory error is reduced by calibration but NOT by running replicates?
- Random error
- Systematic error (Correct answer)
- Gross error
- Indeterminate error
Correct answer: Systematic error
Systematic (determinate) error affects all measurements in the same direction and is corrected through calibration, not replication.
Question 7: In a UV-Vis spectrum, the molar absorptivity (ε) is highest for which type of electronic transition?
- n→σ*
- σ→σ*
- n→π*
- π→π* (Correct answer)
Correct answer: π→π*
π→π* transitions are symmetry-allowed and have high molar absorptivities (ε > 10,000 L·mol⁻¹·cm⁻¹), especially in conjugated systems.
ACS CP Chemical Principles & Laboratory Techniques 2
Which colligative property is used to determine the molar mass of a solute via osmotic pressure measurement?