Preparing a standard solution is a fundamental skill in analytical chemistry that every student must master. This comprehensive guide covers the preparation of a standard solution of oxalic acid through a detailed laboratory experiment verification.
To prepare a standard solution of oxalic acid (0.1M) and verify its concentration through titration with standardized sodium hydroxide solution.
A standard solution is a solution whose concentration is precisely known. It serves as a reference for quantitative analysis in titrations and other analytical procedures.
Oxalic acid dihydrate is chosen for standard solution preparation because:
H₂C₂O₄ + 2NaOH → Na₂C₂O₄ + 2H₂O
The reaction shows that one mole of oxalic acid reacts with two moles of sodium hydroxide.
Molarity (M) = (Weight of solute × 1000) / (Molecular weight × Volume in mL)
For oxalic acid dihydrate: Molecular weight = 126 g/mol
Weighing: Accurately weigh about 1.575 g of oxalic acid dihydrate crystals in a clean, dry weighing bottle using an analytical balance.
Transfer: Transfer the weighed crystals carefully into a clean 250 mL volumetric flask through a funnel.
Washing: Wash the weighing bottle and funnel with distilled water, ensuring all crystals are transferred to the flask.
Dissolution: Add about 150 mL of distilled water to the flask and shake well to dissolve the crystals completely.
Volume Adjustment: Add more distilled water slowly up to the calibration mark of the volumetric flask. The bottom of the meniscus should touch the mark.
Mixing: Stopper the flask and invert it several times to ensure uniform mixing.
Pipette: Using a pipette, transfer exactly 25 mL of the prepared oxalic acid solution into a clean conical flask.
Indicator: Add 2-3 drops of phenolphthalein indicator to the solution.
Burette Setup: Fill the burette with standardized 0.1M NaOH solution and record the initial reading.
Titration: Slowly add NaOH solution from the burette to the conical flask while constantly swirling. Continue until a permanent pink color appears.
End Point: The endpoint is reached when the pink color persists for at least 30 seconds.
Repeat: Perform the titration three times and record the readings.
| Titration No. | Initial Burette Reading (mL) | Final Burette Reading (mL) | Volume of NaOH used (mL) |
|---|---|---|---|
| 1 | 0.00 | 24.80 | 24.80 |
| 2 | 0.00 | 24.85 | 24.85 |
| 3 | 0.00 | 24.82 | 24.82 |
Concordant Reading: 24.82 mL (average of three readings)
Expected Molarity of Oxalic Acid Solution:
Verification using Titration Data:
Conclusion: The prepared oxalic acid solution has a molarity of 0.05M, confirming successful preparation of standard solution.
Accurate Weighing: Use analytical balance for precise weighing of oxalic acid crystals.
Proper Transfer: Ensure complete transfer of crystals to avoid concentration errors.
Meniscus Reading: Always read the burette at eye level, taking the bottom of the meniscus.
End Point Detection: Add NaOH drop by drop near the endpoint for accurate results.
Clean Glassware: Use clean and dry glassware to prevent contamination.
Proper Mixing: Mix the final solution thoroughly by inverting the volumetric flask multiple times.
Storage: Store the prepared solution in a properly labeled bottle.
A: It is a primary standard substance, available in pure form, stable in air, and has high molecular weight reducing weighing errors.
A: The molecular weight is 126 g/mol (H₂C₂O₄·2H₂O).
A: Thorough mixing ensures uniform concentration throughout the solution.
A: Meniscus provides accurate volume measurement as liquids form a curved surface in glass containers.
A: To prevent any additional liquid from the funnel from affecting the final volume measurement.
A: Molarity = (Weight × 1000) / (Molecular weight × Volume in mL)
A: It changes color from colorless to pink at the endpoint, indicating completion of neutralization reaction.
A: Distilled water is free from impurities that could interfere with the analysis or affect solution concentration.
This comprehensive guide provides everything needed for successful preparation and verification of a standard oxalic acid solution. The experiment demonstrates fundamental principles of analytical chemistry while developing essential laboratory skills for chemistry students.
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