Chemistry Test Analytical Chemistry 3 ā Questions and Answers
Question 1: In HPLC, what distinguishes reverse-phase from normal-phase chromatography?
- Reverse-phase uses a polar stationary phase and nonpolar mobile phase
- Reverse-phase uses a nonpolar stationary phase (e.g., C18) and a polar mobile phase (Correct answer)
- Normal-phase uses aqueous buffers exclusively
- Reverse-phase operates at higher pressures than normal-phase
Correct answer: Reverse-phase uses a nonpolar stationary phase (e.g., C18) and a polar mobile phase
Reverse-phase HPLC uses a nonpolar stationary phase (like C18 silica) and a polar aqueous/organic mobile phase, opposite to normal-phase.
Question 2: What is the purpose of an internal standard in quantitative analytical chemistry?
- To calibrate the instrument wavelength
- To compensate for variations in sample preparation and instrument response (Correct answer)
- To act as a blank for background subtraction
- To increase the solubility of the analyte
Correct answer: To compensate for variations in sample preparation and instrument response
An internal standard is added in a known amount to correct for losses or variability during sample preparation and analysis.
Question 3: Which method of standard addition is most useful when the sample matrix interferes with the analytical signal?
- External calibration
- Standard addition method (Correct answer)
- Blank subtraction
- Dilution to extinction
Correct answer: Standard addition method
The standard addition method adds known quantities of analyte to the sample, accounting for matrix effects that external calibration cannot address.
Question 4: In potentiometry, the Nernst equation relates electrode potential to which variable?
- Temperature only
- Ion activity (concentration) (Correct answer)
- Current flow
- Resistance of the solution
Correct answer: Ion activity (concentration)
The Nernst equation (E = EĀ° ā (RT/nF)ln Q) shows that electrode potential depends on the activity (effectively concentration) of the ion.
Question 5: What is the limit of detection (LOD) in analytical chemistry?
- The highest concentration that can be measured without dilution
- The smallest amount of analyte that can be reliably distinguished from a blank (Correct answer)
- The concentration at which the calibration curve becomes nonlinear
- The precision of replicate measurements at mid-range concentration
Correct answer: The smallest amount of analyte that can be reliably distinguished from a blank
LOD is the minimum analyte quantity that produces a signal distinguishable from background noise, typically defined as blank signal plus 3 standard deviations.
Question 6: Which spectroscopic technique identifies functional groups in organic molecules by measuring absorption of infrared radiation?
- UV-Vis spectroscopy
- Infrared (IR) spectroscopy (Correct answer)
- Fluorescence spectroscopy
- Raman spectroscopy
Correct answer: Infrared (IR) spectroscopy
IR spectroscopy measures absorption of infrared light corresponding to molecular bond vibrations, revealing functional groups.
Question 7: What does a high resolution mass spectrum uniquely provide compared to a low resolution spectrum?
- Higher sensitivity for trace analytes
- Exact molecular mass to identify molecular formula (Correct answer)
- Better fragmentation patterns
- Broader m/z range coverage
Correct answer: Exact molecular mass to identify molecular formula
High resolution mass spectrometry provides exact masses with enough precision to determine the elemental composition (molecular formula) of ions.
Chemistry Test Analytical Chemistry 3
In HPLC, what distinguishes reverse-phase from normal-phase chromatography?