STP Laboratory Practice 5 — Questions and Answers
Question 1: A scientist needs to prepare a 100 mM NaCl solution. If the molecular weight of NaCl is 58.44 g/mol, how many grams are needed to make 500 mL of this solution?
- 2.922 g (Correct answer)
- 5.844 g
- 29.22 g
- 0.5844 g
Correct answer: 2.922 g
Moles needed = 0.1 mol/L × 0.5 L = 0.05 mol; mass = 0.05 mol × 58.44 g/mol = 2.922 g.
Question 2: Which of the following is the correct hierarchy of controls for managing laboratory hazards, from most to least preferred?
- PPE → Administrative controls → Engineering controls → Substitution → Elimination
- Elimination → Substitution → Engineering controls → Administrative controls → PPE (Correct answer)
- Administrative controls → PPE → Engineering controls → Substitution → Elimination
- Engineering controls → Elimination → PPE → Administrative controls → Substitution
Correct answer: Elimination → Substitution → Engineering controls → Administrative controls → PPE
The hierarchy of controls prioritizes eliminating the hazard first, then substitution, engineering controls, administrative controls, and lastly PPE as the least reliable protection.
Question 3: In a double-blind clinical trial, which of the following BEST describes the role of laboratory staff processing samples?
- Laboratory staff must know group assignments to process samples correctly
- Laboratory staff should be blinded to sample group assignment to prevent bias in processing or analysis (Correct answer)
- Only the principal investigator needs to be blinded
- Blinding applies only to patients, not laboratory personnel
Correct answer: Laboratory staff should be blinded to sample group assignment to prevent bias in processing or analysis
In a double-blind study, laboratory personnel are also blinded to treatment assignments to prevent conscious or unconscious bias in sample handling and result interpretation.
Question 4: What does the term 'limit of detection' (LOD) mean in analytical chemistry?
- The highest concentration that can be measured accurately
- The lowest quantity of an analyte that can be distinguished from background with statistical confidence (Correct answer)
- The concentration range over which the assay is linear
- The minimum sample volume required for a valid measurement
Correct answer: The lowest quantity of an analyte that can be distinguished from background with statistical confidence
The LOD is the smallest amount of analyte that produces a signal distinguishable from background noise, typically defined as signal-to-noise ratio of 3:1.
Question 5: A researcher is working with a Category B infectious substance for shipping. Under US DOT regulations, which packaging specification must be used?
- UN2814 triple packaging with primary, secondary, and rigid outer packaging
- P650 packaging specification for UN3373 (Correct answer)
- Any leak-proof container with a biohazard label
- Standard postal packaging with a biological specimen label
Correct answer: P650 packaging specification for UN3373
Category B infectious substances classified as UN3373 must be shipped using P650 packaging, which requires primary, secondary, and outer containers meeting specific requirements.
Question 6: Which statistical term describes the closeness of agreement between independent test results obtained under the same conditions (same operator, same equipment, short time interval)?
- Reproducibility
- Accuracy
- Repeatability (Correct answer)
- Selectivity
Correct answer: Repeatability
Repeatability (intra-assay precision) measures agreement between results obtained by the same analyst using the same equipment in a short timeframe; reproducibility applies across different conditions.
Question 7: When storing flammable solvents in a laboratory, which practice is REQUIRED for safety?
- Storing them in a standard refrigerator to keep them cool
- Keeping all flammable liquids in an approved flammable storage cabinet away from ignition sources (Correct answer)
- Storing them near a sink for easy access to water in case of fire
- Keeping them in the fume hood at all times for ventilation
Correct answer: Keeping all flammable liquids in an approved flammable storage cabinet away from ignition sources
Flammable liquids must be stored in approved flammable storage cabinets designed to contain vapors and withstand fire, away from ignition sources.
STP Laboratory Practice 5
A scientist needs to prepare a 100 mM NaCl solution.
If the molecular weight of NaCl is 58.44 g/mol, how many grams are needed to make 500 mL of this solution?