The given solution contains 6gms of Na2CO3 dissolved per dm3. Determine the Percentage Purity of the Sample Solution by Volumetric Method

Abstract:

This experiment focuses on the determination of the percentage purity of a sample solution of Na₂CO₃ using volumetric analysis. The experiment involves titrating the Na₂CO₃ solution with a standard solution of HCl using phenolphthalein as an indicator. The volume of HCl used in titration is used to calculate the percentage purity of the sample solution.

Introduction:

Volumetric analysis, also known as titration, is a common technique used in chemistry to determine the concentration or purity of a substance in a solution. In this experiment, we will focus on the determination of the percentage purity of a sample solution of Na₂CO₃ (sodium carbonate) using volumetric analysis.

The concept of titration involves the reaction between an analyte (the substance being analyzed) and a titrant (the solution of known concentration). By measuring the volume of titrant required to react completely with the analyte, we can determine the concentration or purity of the analyte.

Phenolphthalein is commonly used as an indicator in acid-base titrations because it changes color at the pH range corresponding to the equivalence point of the reaction.

Objective:

To determine the percentage purity of a sample solution of Na₂CO₃ using volumetric analysis.

Materials Required:

• Standard solution of HCl
• Phenolphthalein indicator
• Na₂CO₃ sample solution
• Burette
• Pipette
• Conical flask
• Beaker
• Distilled water

Procedure:

1. Using a pipette, transfer a known volume (e.g., 25 mL) of the Na₂CO₃ sample solution into a conical flask.
2. Add a few drops of phenolphthalein indicator to the conical flask.
3. Titrate the Na₂CO₃ solution with the standard HCl solution from the burette until a permanent pink color is obtained.
4. Note down the volume of HCl used.

Calculation:

Let \( V_1 \) be the volume of HCl used in titration (in mL).

Let \( N_{\text{HCl}} \) be the normality of HCl solution.

Let \( M_{\text{Na}_2\text{CO}_3} \) be the molar mass of Na₂CO₃.

Let \( V_{\text{sample}} \) be the volume of the Na₂CO₃ sample solution used (in mL).

Then, the percentage purity of the sample solution can be calculated using the formula:

\[ \text{Percentage purity} = \left( \frac{{V_1 \times N_{\text{HCl}} \times M_{\text{Na}_2\text{CO}_3}}}{{V_{\text{sample}}}} \right) \times 100 \]

Questions:

1. What is the objective of this experiment?

Answer: The objective is to determine the percentage purity of a sample solution of Na₂CO₃ using volumetric analysis.

2. Why is phenolphthalein used as an indicator in this experiment?

Answer: Phenolphthalein is used because it changes color at the pH range corresponding to the equivalence point of the titration, making it easy to determine when the reaction is complete.

3. What is the role of distilled water in this experiment?

Answer: Distilled water is used for dilution and rinsing of glassware to ensure accurate measurements and prevent contamination.

4. Explain the significance of using a standardized solution in titration.

Answer: A standardized solution has a known concentration, allowing for precise and accurate determination of the unknown concentration or purity of another substance through titration.

5. What precautions should be taken during the experiment to ensure accuracy?

Answer: Precautions include proper calibration of equipment, accurate measurement of volumes, avoiding contamination, and ensuring proper mixing of solutions.

6. What factors can affect the accuracy of the experiment?

Answer: Factors such as impurities in reagents, temperature variations, improper mixing, and equipment calibration can affect the accuracy of the experiment.

7. How does the molar mass of Na₂CO₃ affect the calculation of percentage purity?

Answer: The molar mass is used to convert volume-based measurements to mass-based measurements, which is necessary for calculating percentage purity.

8. How does the volume of the Na₂CO₃ sample solution affect the calculation of percentage purity?

Answer: The volume of the sample solution is used in the calculation to determine the amount of substance present in the sample, affecting the percentage purity calculation.

9. What would happen if too much phenolphthalein indicator is added?

Answer: Excess phenolphthalein can cause the solution to become too dark, making it difficult to observe the color change at the endpoint of the titration.

10. What is the purpose of rinsing the burette with the standard solution before filling it?

Answer: Rinsing removes any residual solution or contaminants, ensuring that the subsequent solution added to the burette is not contaminated and does not affect the accuracy of the titration.

11. What is the purpose of swirling the conical flask during titration?

Answer: Swirling ensures uniform mixing of reactants and helps to achieve a homogenous solution, ensuring the accuracy of the titration.

12. What is the purpose of using a white tile under the conical flask during titration?

Answer: A white tile provides a contrasting background, making it easier to observe color changes in the solution, particularly at the endpoint of the titration.

13. How would you know if the titration is complete?

Answer: The titration is complete when the phenolphthalein indicator changes color permanently, indicating that the reaction between the analyte and titrant is finished.

14. What is the function of the pipette in this experiment?

Answer: The pipette is used to accurately measure and transfer a known volume of the sample solution into the conical flask.

15. Why is it important to record the initial and final readings of the burette?

Answer: Recording the initial and final readings allows for the calculation of the volume of titrant used in the titration, which is essential for determining the concentration or purity of the analyte.

16. How does the concentration of the standard solution affect the accuracy of the experiment?

Answer: Higher concentrations of the standard solution can lead to faster reactions and more significant color changes, making it easier to determine the endpoint of the titration accurately.

17. What is the purpose of using a conical flask instead of a beaker for titration?

Answer: Conical flasks allow for better mixing of solutions due to their narrower necks and are designed to facilitate swirling, ensuring more accurate and reliable results compared to beakers.

18. How does temperature affect the volume of a liquid?

Answer: Generally, as temperature increases, the volume of a liquid expands due to thermal expansion, leading to a larger measured volume. However, this effect is relatively small for small temperature changes.

19. What would happen if the Na₂CO₃ sample solution was not properly mixed before pipetting?

Answer: Improper mixing can lead to non-uniform distribution of the solute in the solution, resulting in inaccurate volume measurements and affecting the accuracy of the titration.

20. What is the purpose of performing a blank titration?

Answer: A blank titration helps to account for any impurities or contaminants present in the solvent or reagents used in the experiment, ensuring that only the analyte's contribution is considered in the calculation.

Multiple Choice Questions:

1. What is the purpose of adding phenolphthalein to the conical flask during the experiment?
• a) To neutralize the solution
• b) To indicate the endpoint of the titration
• c) To speed up the reaction
• d) To increase the accuracy of the experiment

Answer: b) To indicate the endpoint of the titration

2. What is the color change observed when phenolphthalein indicates the endpoint of the titration?
• a) Yellow to red
• b) Pink to colorless
• c) Colorless to pink
• d) Blue to green

Answer: c) Colorless to pink

3. Which factor does NOT affect the accuracy of the experiment?
• a) Volume of HCl used
• b) Concentration of Na₂CO₃ solution
• c) Temperature of the solutions
• d) Quality of glassware

Answer: d) Quality of glassware

4. What is the purpose of using a conical flask instead of a beaker for titration?
• a) Conical flasks are cheaper
• b) Conical flasks are easier to clean
• c) Conical flasks allow for better mixing of solutions
• d) Conical flasks have higher accuracy

Answer: c) Conical flasks allow for better mixing of solutions

5. What would happen if the Na₂CO₃ sample solution was not properly mixed before pipetting?
• a) The color change would not occur
• b) The titration would be too slow
• c) The titrant volume would be inaccurate
• d) The solution would become too acidic

Answer: c) The titrant volume would be inaccurate