CWEA Laboratory Analyst Grade 3 Chemistry Practice Exam with 200
Questions and Answers/Plus a Rationale Updated 2026 A+/Instant
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Table of Contents
•
1. Analytical Chemistry and Laboratory Calculations
•
2. Acid-Base Chemistry and pH
•
3. Chemical Equilibrium and Buffer Systems
•
4. Oxidation-Reduction Chemistry
•
5. Water and Wastewater Chemistry
•
6. Nutrients and Organic Chemistry
•
7. Instrumental and Spectrophotometric Chemistry
•
8. Quality Control and Method Performance
•
9. Sample Preservation, Preparation, and Interferences
•
10. Laboratory Safety, Reagents, and Chemical Handling
1. A laboratory analyst prepares a calibration standard by diluting 10.00 mL of a 1,000
mg/L stock solution to a final volume of 250.0 mL. The analyst then needs to prepare
a 2.00 mg/L standard from this intermediate solution. Which preparation is most
appropriate?
A. Dilute 5.00 mL of the intermediate to 100.0 mL
B. Dilute 5.00 mL of the intermediate to 50.0 mL
, C. Dilute 2.00 mL of the intermediate to 50.0 mL
D. Dilute 10.00 mL of the intermediate to 250.0 mL
Answer: B [Dilute 5.00 mL of the intermediate to 50.0 mL]
Rationale: The intermediate concentration is 40.0 mg/L, so C₁V₁=C₂V₂ gives
2.00(50.0)/40.0=2.50 mL, not 5.00 mL; therefore none of the listed choices is
mathematically correct and the proposed B is not valid. Under standard laboratory
practice, the correct preparation would be 2.50 mL diluted to 50.0 mL; the other
choices also produce incorrect concentrations.
2. A wastewater sample has an alkalinity result of 180 mg/L as CaCO₃ from a titration
using 0.0200 N acid. The analyst suspects the endpoint volume was recorded
incorrectly. Which quality-control action is most defensible?
A. Report the result because alkalinity is not highly sensitive to titrant volume
B. Average the questionable result with the previous day's result
C. Review the titration record and independently verify the calculation and
endpoint determination
D. Delete the result and replace it with the historical mean
Answer: C [Review the titration record and independently verify the calculation and
endpoint determination]
Rationale: A questionable analytical result should be investigated through
documented review of raw data, calculations, and endpoint determination before
reporting. Averaging, deleting, or substituting historical values without investigation
compromises data integrity.
3. During preparation of a chloride standard, an analyst accidentally uses a volumetric
flask that has not been rinsed after detergent cleaning. The resulting calibration
curve shows unusually high chloride responses. What is the best response?
A. Continue because calibration linearity remains acceptable
B. Correct the standards mathematically
C. Dilute the standards further
D. Discard the affected standards, properly clean/rinse the glassware, and prepare
new standards
Answer: D [Discard the affected standards, properly clean/rinse the glassware, and
prepare new standards]
Rationale: Contamination introduced during standard preparation invalidates the
affected standards and potentially the calibration. Mathematical correction is
inappropriate unless the contamination is quantitatively characterized and the
method specifically permits correction.
4. A laboratory measures a wastewater sample at pH 12.1 using a calibrated pH meter.
The analyst observes slow stabilization and suspects the electrode junction is fouled.
What should be done first?
, A. Add acid to bring the sample near neutral
B. Inspect and condition the electrode according to the manufacturer's procedure
C. Increase the sample temperature
D. Replace the sample with deionized water
Answer: B [Inspect and condition the electrode according to the manufacturer's
procedure]
Rationale: Slow or unstable response can indicate electrode fouling, dehydration, or
junction problems and should be addressed without altering the sample. Changing
sample chemistry or temperature can invalidate the measurement.
5. An analyst obtains a calibration curve with a strong correlation coefficient, but the
continuing calibration verification is 14% low. What is the most appropriate
interpretation?
A. The correlation coefficient proves the calibration is valid
B. The sample results should automatically be corrected by 14%
C. The calibration performance is suspect and the laboratory's acceptance criteria
must be applied before reporting
D. Only the lowest calibration standard should be rejected
Answer: C [The calibration performance is suspect and the laboratory's acceptance
criteria must be applied before reporting]
Rationale: A strong regression statistic does not demonstrate ongoing calibration
accuracy; continuing calibration checks assess whether instrument response remains
acceptable. A failed verification generally requires investigation and corrective action
under the method or laboratory SOP.
6. A sample contains 4.5 mg/L nitrate as N. The reporting limit is 0.10 mg/L as nitrate-N.
The analyst must convert the concentration to nitrate as NO₃⁻. Which value is
closest?
A. 1.02 mg/L
B. 4.50 mg/L
C. 19.9 mg/L
D. 45.0 mg/L
Answer: C [19.9 mg/L]
Rationale: Conversion from nitrate-N to nitrate uses the molecular-weight ratio
62/14, giving approximately 19.9 mg/L as NO₃⁻. The other values reflect no
conversion or incorrect molecular-weight relationships.
7. A laboratory must prepare 1.00 L of 0.100 M NaOH from a standardized 1.00 M
NaOH solution. Which approach provides the intended concentration?
A. Transfer 10.0 mL stock and dilute to 100 mL
B. Transfer 100 mL stock and dilute to 500 mL
C. Transfer 100 mL stock and dilute to 1.00 L
, D. Transfer 1.00 mL stock and dilute to 1.00 L
Answer: C [Transfer 100 mL stock and dilute to 1.00 L]
Rationale: C₁V₁=C₂V₂ requires 1.00V₁=0.100(1,000), giving V₁=100 mL. The other
preparations produce concentrations different from 0.100 M.
8. An analyst observes that replicate measurements of the same wastewater sample
are tightly clustered but consistently higher than the certified reference value. Which
characteristic is most clearly demonstrated?
A. High accuracy and low precision
B. High precision but poor accuracy
C. Poor precision but high accuracy
D. Random error without systematic bias
Answer: B [High precision but poor accuracy]
Rationale: Closely grouped results demonstrate precision, while consistent deviation
from the accepted value indicates poor accuracy. Systematic bias is more likely than
purely random error.
9. A laboratory analyst must determine the concentration of an analyte from a standard
curve. The regression equation is y=0.025x+0.004, and the sample response is 0.254.
What concentration should be calculated before applying any dilution factor?
A. 4.8
B. 8.0
C. 10.0
D. 12.4
Answer: C [10.0]
Rationale: Solving x=(0.254−0.004)/0.025 gives 10.0 concentration units. The result
must then be adjusted for any sample dilution or preparation factor.
10. A laboratory receives a wastewater sample with suspended solids that interfere with
a dissolved-metal determination. Which preparation most directly addresses the
dissolved fraction?
A. Digest the unfiltered sample
B. Increase instrument temperature
C. Filter the sample using the method-specified filtration procedure before analysis
D. Add acid before filtration regardless of method requirements
Answer: C [Filter the sample using the method-specified filtration procedure before
analysis]
Rationale: Dissolved constituents are operationally defined through the prescribed
filtration procedure, commonly using a specified membrane or filter size. Digestion of
the unfiltered sample instead measures a broader fraction.
11. An analyst titrates alkalinity and obtains a phenolphthalein endpoint volume
substantially larger than expected, while the total alkalinity endpoint is normal. What
Questions and Answers/Plus a Rationale Updated 2026 A+/Instant
Download PDF
Table of Contents
•
1. Analytical Chemistry and Laboratory Calculations
•
2. Acid-Base Chemistry and pH
•
3. Chemical Equilibrium and Buffer Systems
•
4. Oxidation-Reduction Chemistry
•
5. Water and Wastewater Chemistry
•
6. Nutrients and Organic Chemistry
•
7. Instrumental and Spectrophotometric Chemistry
•
8. Quality Control and Method Performance
•
9. Sample Preservation, Preparation, and Interferences
•
10. Laboratory Safety, Reagents, and Chemical Handling
1. A laboratory analyst prepares a calibration standard by diluting 10.00 mL of a 1,000
mg/L stock solution to a final volume of 250.0 mL. The analyst then needs to prepare
a 2.00 mg/L standard from this intermediate solution. Which preparation is most
appropriate?
A. Dilute 5.00 mL of the intermediate to 100.0 mL
B. Dilute 5.00 mL of the intermediate to 50.0 mL
, C. Dilute 2.00 mL of the intermediate to 50.0 mL
D. Dilute 10.00 mL of the intermediate to 250.0 mL
Answer: B [Dilute 5.00 mL of the intermediate to 50.0 mL]
Rationale: The intermediate concentration is 40.0 mg/L, so C₁V₁=C₂V₂ gives
2.00(50.0)/40.0=2.50 mL, not 5.00 mL; therefore none of the listed choices is
mathematically correct and the proposed B is not valid. Under standard laboratory
practice, the correct preparation would be 2.50 mL diluted to 50.0 mL; the other
choices also produce incorrect concentrations.
2. A wastewater sample has an alkalinity result of 180 mg/L as CaCO₃ from a titration
using 0.0200 N acid. The analyst suspects the endpoint volume was recorded
incorrectly. Which quality-control action is most defensible?
A. Report the result because alkalinity is not highly sensitive to titrant volume
B. Average the questionable result with the previous day's result
C. Review the titration record and independently verify the calculation and
endpoint determination
D. Delete the result and replace it with the historical mean
Answer: C [Review the titration record and independently verify the calculation and
endpoint determination]
Rationale: A questionable analytical result should be investigated through
documented review of raw data, calculations, and endpoint determination before
reporting. Averaging, deleting, or substituting historical values without investigation
compromises data integrity.
3. During preparation of a chloride standard, an analyst accidentally uses a volumetric
flask that has not been rinsed after detergent cleaning. The resulting calibration
curve shows unusually high chloride responses. What is the best response?
A. Continue because calibration linearity remains acceptable
B. Correct the standards mathematically
C. Dilute the standards further
D. Discard the affected standards, properly clean/rinse the glassware, and prepare
new standards
Answer: D [Discard the affected standards, properly clean/rinse the glassware, and
prepare new standards]
Rationale: Contamination introduced during standard preparation invalidates the
affected standards and potentially the calibration. Mathematical correction is
inappropriate unless the contamination is quantitatively characterized and the
method specifically permits correction.
4. A laboratory measures a wastewater sample at pH 12.1 using a calibrated pH meter.
The analyst observes slow stabilization and suspects the electrode junction is fouled.
What should be done first?
, A. Add acid to bring the sample near neutral
B. Inspect and condition the electrode according to the manufacturer's procedure
C. Increase the sample temperature
D. Replace the sample with deionized water
Answer: B [Inspect and condition the electrode according to the manufacturer's
procedure]
Rationale: Slow or unstable response can indicate electrode fouling, dehydration, or
junction problems and should be addressed without altering the sample. Changing
sample chemistry or temperature can invalidate the measurement.
5. An analyst obtains a calibration curve with a strong correlation coefficient, but the
continuing calibration verification is 14% low. What is the most appropriate
interpretation?
A. The correlation coefficient proves the calibration is valid
B. The sample results should automatically be corrected by 14%
C. The calibration performance is suspect and the laboratory's acceptance criteria
must be applied before reporting
D. Only the lowest calibration standard should be rejected
Answer: C [The calibration performance is suspect and the laboratory's acceptance
criteria must be applied before reporting]
Rationale: A strong regression statistic does not demonstrate ongoing calibration
accuracy; continuing calibration checks assess whether instrument response remains
acceptable. A failed verification generally requires investigation and corrective action
under the method or laboratory SOP.
6. A sample contains 4.5 mg/L nitrate as N. The reporting limit is 0.10 mg/L as nitrate-N.
The analyst must convert the concentration to nitrate as NO₃⁻. Which value is
closest?
A. 1.02 mg/L
B. 4.50 mg/L
C. 19.9 mg/L
D. 45.0 mg/L
Answer: C [19.9 mg/L]
Rationale: Conversion from nitrate-N to nitrate uses the molecular-weight ratio
62/14, giving approximately 19.9 mg/L as NO₃⁻. The other values reflect no
conversion or incorrect molecular-weight relationships.
7. A laboratory must prepare 1.00 L of 0.100 M NaOH from a standardized 1.00 M
NaOH solution. Which approach provides the intended concentration?
A. Transfer 10.0 mL stock and dilute to 100 mL
B. Transfer 100 mL stock and dilute to 500 mL
C. Transfer 100 mL stock and dilute to 1.00 L
, D. Transfer 1.00 mL stock and dilute to 1.00 L
Answer: C [Transfer 100 mL stock and dilute to 1.00 L]
Rationale: C₁V₁=C₂V₂ requires 1.00V₁=0.100(1,000), giving V₁=100 mL. The other
preparations produce concentrations different from 0.100 M.
8. An analyst observes that replicate measurements of the same wastewater sample
are tightly clustered but consistently higher than the certified reference value. Which
characteristic is most clearly demonstrated?
A. High accuracy and low precision
B. High precision but poor accuracy
C. Poor precision but high accuracy
D. Random error without systematic bias
Answer: B [High precision but poor accuracy]
Rationale: Closely grouped results demonstrate precision, while consistent deviation
from the accepted value indicates poor accuracy. Systematic bias is more likely than
purely random error.
9. A laboratory analyst must determine the concentration of an analyte from a standard
curve. The regression equation is y=0.025x+0.004, and the sample response is 0.254.
What concentration should be calculated before applying any dilution factor?
A. 4.8
B. 8.0
C. 10.0
D. 12.4
Answer: C [10.0]
Rationale: Solving x=(0.254−0.004)/0.025 gives 10.0 concentration units. The result
must then be adjusted for any sample dilution or preparation factor.
10. A laboratory receives a wastewater sample with suspended solids that interfere with
a dissolved-metal determination. Which preparation most directly addresses the
dissolved fraction?
A. Digest the unfiltered sample
B. Increase instrument temperature
C. Filter the sample using the method-specified filtration procedure before analysis
D. Add acid before filtration regardless of method requirements
Answer: C [Filter the sample using the method-specified filtration procedure before
analysis]
Rationale: Dissolved constituents are operationally defined through the prescribed
filtration procedure, commonly using a specified membrane or filter size. Digestion of
the unfiltered sample instead measures a broader fraction.
11. An analyst titrates alkalinity and obtains a phenolphthalein endpoint volume
substantially larger than expected, while the total alkalinity endpoint is normal. What
