Preparation and standardization of sodium hydroxide solution MCQs With Answer

Preparation and standardization of sodium hydroxide solution MCQs With Answer

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Introduction: Preparation and standardization of sodium hydroxide solution is a core practical skill for B.Pharm students, covering NaOH solution preparation, titration technique, primary standard selection, and concentration calculation. This topic emphasizes handling hygroscopic NaOH, avoiding CO2 absorption, choosing a primary standard such as potassium hydrogen phthalate (KHP), and using indicators like phenolphthalein for accurate end-point detection. Students learn calibration of volumetric glassware, calculation of molarity/normality, common errors, and safety precautions for this corrosive alkali. Mastery of these principles ensures reliable assays in pharmaceutical analysis and quality control. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the main reason sodium hydroxide pellets cannot be accurately used to prepare a standard solution by direct weighing?

  • NaOH pellets are too expensive
  • NaOH pellets are hygroscopic and absorb moisture and CO2
  • NaOH pellets react explosively with water
  • NaOH pellets decompose into sodium carbonate on heating

Correct Answer: NaOH pellets are hygroscopic and absorb moisture and CO2

Q2. Which compound is most commonly used as a primary standard to standardize NaOH?

  • Sodium chloride
  • Potassium hydrogen phthalate (KHP)
  • Sulfuric acid
  • Sodium carbonate

Correct Answer: Potassium hydrogen phthalate (KHP)

Q3. Which indicator is typically used when standardizing NaOH with KHP?

  • Methyl orange
  • Phenolphthalein
  • Bromothymol blue
  • Universal indicator paper

Correct Answer: Phenolphthalein

Q4. Which property is essential for a substance to be a good primary standard?

  • Hygroscopic and impure
  • High volatility and reactive with air
  • High purity, stable, non-hygroscopic and known molar mass
  • Colored and slightly soluble

Correct Answer: High purity, stable, non-hygroscopic and known molar mass

Q5. During the standardization titration of NaOH with KHP, the endpoint indicated by a persistent faint pink color corresponds to what?

  • Excess KHP in the flask
  • Equivalence point where moles of OH- equal moles of KHP acid equivalents
  • The solution is neutral (pH 7)
  • Complete decomposition of KHP

Correct Answer: Equivalence point where moles of OH- equal moles of KHP acid equivalents

Q6. KHP (potassium hydrogen phthalate) has a molar mass ≈ 204.22 g·mol−1. If 0.20422 g of KHP is titrated to endpoint, how many moles of KHP were used?

  • 1.00 × 10−2 mol
  • 1.00 × 10−3 mol
  • 2.04 × 10−3 mol
  • 1.00 × 10−4 mol

Correct Answer: 1.00 × 10−3 mol

Q7. If 0.20422 g KHP (MW 204.22 g·mol−1) neutralizes NaOH and the titration used 25.00 mL NaOH, what is the molarity of NaOH?

  • 0.0400 M
  • 0.00400 M
  • 0.400 M
  • 0.0160 M

Correct Answer: 0.0400 M

Q8. Why is CO2 absorption a concern when preparing and storing NaOH solutions?

  • CO2 converts NaOH to sodium carbonate, lowering effective OH− concentration
  • CO2 causes NaOH to become more basic
  • CO2 reacts to form explosive products with NaOH
  • CO2 prevents NaOH from dissolving

Correct Answer: CO2 converts NaOH to sodium carbonate, lowering effective OH− concentration

Q9. Which of the following handling steps helps minimize CO2 uptake during NaOH solution preparation?

  • Boiling NaOH solution for 30 minutes uncovered
  • Preparing and standardizing solution immediately and storing in tightly stoppered bottles
  • Leaving the solution in a beaker to cool in air
  • Adding concentrated HCl to the NaOH solution

Correct Answer: Preparing and standardizing solution immediately and storing in tightly stoppered bottles

Q10. In titration terminology, why is NaOH often called the titrant?

  • Because it is the substance of unknown concentration added from the burette
  • Because it is always the analyte in pharmaceutical assays
  • Because it is less reactive than acids
  • Because it is colored and visible

Correct Answer: Because it is the substance of unknown concentration added from the burette

Q11. Which glassware accuracy is most appropriate for preparing a standard NaOH solution of known molarity?

  • Graduated cylinder
  • Volumetric flask
  • Beaker
  • Erlenmeyer flask

Correct Answer: Volumetric flask

Q12. Normality and molarity of NaOH are equivalent because:

  • NaOH is diprotic
  • NaOH provides one OH− per formula unit (monoprotic behavior)
  • Normality is always equal to molarity for all solutions
  • NaOH reacts with water to form two equivalents

Correct Answer: NaOH provides one OH− per formula unit (monoprotic behavior)

Q13. Which error will lead to an erroneously low calculated concentration of NaOH after standardization?

  • Using too much indicator
  • Under-titrating (stopping before the true endpoint)
  • Using a primary standard of known purity
  • Rinsing the burette with the titrant before filling

Correct Answer: Under-titrating (stopping before the true endpoint)

Q14. What is the typical pH at equivalence for titration of KHP (weak acid) with NaOH (strong base)?

  • Approximately 7.0
  • Acidic (<7)
  • Basic (>7)
  • Exactly 14

Correct Answer: Basic (>7)

Q15. Which step is essential before weighing KHP for standardization?

  • Dissolve KHP in NaOH first
  • Dry KHP to constant weight if required and use a dry weighing bottle
  • Expose KHP to air for equilibration
  • React KHP with HCl

Correct Answer: Dry KHP to constant weight if required and use a dry weighing bottle

Q16. If 0.5100 g KHP (MW 204.22 g·mol−1) was titrated and consumed 0.03000 L of NaOH, what is the molarity of NaOH? (Moles KHP = mass/MW)

  • 0.0833 M
  • 0.00833 M
  • 0.08333 M
  • 0.0100 M

Correct Answer: 0.08333 M

Q17. What is a practical precaution when filling the burette with NaOH solution?

  • Fill it completely to the top and leave uncapped
  • Rinse the burette with distilled water only
  • Rinse the burette with the NaOH solution first, then fill and remove air bubbles from the tip
  • Use a cotton plug at the top to allow CO2 diffusion

Correct Answer: Rinse the burette with the NaOH solution first, then fill and remove air bubbles from the tip

Q18. During titration, why is swirling the Erlenmeyer flask important?

  • To cool the solution
  • To ensure thorough mixing of titrant and analyte for accurate reaction and uniform indicator color
  • To evaporate solvent quickly
  • To keep CO2 from dissolving

Correct Answer: To ensure thorough mixing of titrant and analyte for accurate reaction and uniform indicator color

Q19. Which analytical balance practice improves accuracy when weighing reagents like KHP?

  • Weigh on a dirty balance pan
  • Use weigh paper or a dry container and perform repeat weighings to constant mass
  • Weigh reagents while holding the container with fingers
  • Weigh immediately after placing on balance without stabilization

Correct Answer: Use weigh paper or a dry container and perform repeat weighings to constant mass

Q20. Which of the following indicates that NaOH solution has been contaminated by CO2?

  • Solution becomes clearer
  • Solution develops a white precipitate on cooling (sodium carbonate) and requires more NaOH to reach endpoint
  • Solution turns red
  • Solution concentration increases over time

Correct Answer: Solution develops a white precipitate on cooling (sodium carbonate) and requires more NaOH to reach endpoint

Q21. For routine laboratory use, how often should a working NaOH solution be standardized?

  • Never, once prepared it’s always accurate
  • Regularly — daily or before critical assays, depending on exposure and storage
  • Only when color changes
  • Every 10 years

Correct Answer: Regularly — daily or before critical assays, depending on exposure and storage

Q22. Which titration curve feature is expected when titrating a weak acid (KHP) with a strong base (NaOH)?

  • Sharp vertical jump around equivalence with equivalence pH > 7
  • Flat curve with equivalence pH = 7
  • Equivalence pH < 7
  • Continuous linear decrease in pH

Correct Answer: Sharp vertical jump around equivalence with equivalence pH > 7

Q23. When standardizing NaOH, why might you add a few drops of ethanol when dissolving KHP?

  • Ethanol is required to react with NaOH
  • Small amounts improve indicator color stability and wetting of organic solids in some protocols
  • Ethanol neutralizes KHP
  • Ethanol increases the molarity of NaOH

Correct Answer: Small amounts improve indicator color stability and wetting of organic solids in some protocols

Q24. Which calculation is used to find molarity of NaOH after titration with KHP?

  • Molarity NaOH = (mass KHP × volume NaOH) / MW
  • Molarity NaOH = moles KHP / volume NaOH (L)
  • Molarity NaOH = MW × mass KHP / volume NaOH
  • Molarity NaOH = (volume KHP / moles NaOH)

Correct Answer: Molarity NaOH = moles KHP / volume NaOH (L)

Q25. If titration of 0.250 g KHP (MW 204.22 g·mol−1) required 32.50 mL NaOH, what is the molarity of NaOH? (Provide nearest 0.001 M)

  • 0.037 M
  • 0.025 M
  • 0.030 M
  • 0.050 M

Correct Answer: 0.037 M

Q26. What safety precaution is most important when preparing and handling NaOH solutions?

  • Use of personal protective equipment (gloves, goggles, lab coat) and avoid skin contact
  • Keep container open to air for ventilation
  • Heat NaOH solution to boiling before use
  • Store NaOH with acids

Correct Answer: Use of personal protective equipment (gloves, goggles, lab coat) and avoid skin contact

Q27. Which of the following is a sign of overshooting the endpoint during titration?

  • Faint persistent pink appears and remains for 30 seconds
  • Transient color change that disappears upon swirling
  • Solution remains colorless
  • Indicator color becomes much darker than expected and persists

Correct Answer: Indicator color becomes much darker than expected and persists

Q28. Why is it recommended to standardize NaOH using a primary standard rather than relying on manufacturer concentration?

  • Manufacturer concentration never changes
  • Because NaOH solutions change concentration due to hygroscopicity and CO2 absorption, so experimental standardization ensures accuracy
  • Primary standards are cheaper than NaOH
  • Regulations prohibit using manufacturer data

Correct Answer: Because NaOH solutions change concentration due to hygroscopicity and CO2 absorption, so experimental standardization ensures accuracy

Q29. In a typical lab standardization, why is the burette reading recorded to two decimal places (e.g., 12.34 mL)?

  • To make calculations harder
  • To provide sufficient precision for concentration determination and reduce volume measurement error
  • Because digital burettes only display two decimals
  • To match the balance precision

Correct Answer: To provide sufficient precision for concentration determination and reduce volume measurement error

Q30. Which alternative acid could be used as a primary standard to standardize NaOH if KHP is unavailable?

  • Sodium hydroxide itself
  • Oxalic acid dihydrate or benzoic acid (high purity, stable primary standards)
  • Sulfuric acid of unknown purity
  • Hydrochloric acid solution from the lab

Correct Answer: Oxalic acid dihydrate or benzoic acid (high purity, stable primary standards)

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