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Experiment 21 Date: ........................................

PART 1Aim: To accurately determine the concentration of a sodium hydroxide solution by titration against a standard oxalic acid solution.

Apparatus:

• Electronic scale• Small glass cup/evaporator tray• 1 dm3 volumetric flask• Wash bottle• Four Erlenmeyer flasks• Two burettes• Two retort stands• Funnels• White tile

Chemicals:

• A standard oxalic acid solution of about 0,01 mol⋅dm-3

• Sodium hydroxide pellets• Distilled water• Phenolphthalein

Method:

1. Prepare a standard oxalic acid solution with a concentration of 0,01 mol⋅dm-3. Calculate the exact concentration of this solution so that it can be used as the primary standard solution.2. Prepare a NaOH solution with a concentration of about 0,2 mol⋅dm-3 by dissolving about 8 g of NaOH pellets in distilled water and making up the solution to 1 dm3.

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3. Rinse a clean Erlenmeyer flask with distilled water.4. Rinse a clean burette with distilled water and then with a small amount of the NaOH solution.5. Clamp the burette upright in a retort stand.6. Place a funnel in the burette and carefully fill the burette with the NaOH solution.7. Allow the solution to pass through the burette by turning the tap open slightly so that the burette tip is filled with NaOH solution. Keep the waste NaOH solution aside.8. Run a bit of the NaOH solution through the burette so that the initial volume can be read conveniently. Make a note of this reading.9. Rinse the other clean burette with distilled water and then with a small quantity of the prepared standard oxalic acid solution.10. Clamp the burette upright in a retort stand.11. Place a funnel in the burette and carefully fill the burette with the oxalic acid solution.12. Allow the oxalic acid solution to pass through the burette by opening the tap slightly so that the burette tip is filled with oxalic acid solution. Ensure that the initial volume can be read easily and record this value. Keep the waste oxalic acid solution aside.13. Place the Erlenmeyer bottle on the white tile below the burette’s tip.14. Allow exactly 25 cm3 of the oxalic acid solution to run from the burette into the Erlenmeyer flask.15. Add about three drops of phenolphthalein to the oxalic acid solution.16. Titrate with the NaOH solution until the solution’s colour changes to pale pink.17. Take the reading on the NaOH burette. Determine the volume of NaOH solution added to neutralise the standard oxalic acid solution using the initial and end readings on the burette.18. The first result serves as a trial run so that subsequent readings can be taken quickly but accurately.19. Repeat steps 14–17 three more times. The NaOH can now be quickly added to approximately 2 cm3 from the volume added in the trial. Then the NaOH should be added drop by drop so that the end point of the titration and the volume of NaOH added can be accurately determined.20. Record all the data.21. Calculate the average volume of NaOH solution added and use this value to accurately calculate the concentration of the NaOH solution.

Results:

Trial 1 2 3 Average Volume oxalic acid (cm3) 25,00 25,00 25,00 25,00 25,00

Volume NaOH(aq) (cm3)

Balanced reaction equation:

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Calculations:

Concentration of the NaOH solution:

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Conclusions:

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Answer the following questions:

1 The sketch shows the meniscus of a burette for the initial reading of the first run. The final reading is 53,9 mℓ.1.1 How will the results be affected if the burettes are misread as shown in the diagram? Explain your answer in full.

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1.2 What effect does this have on the calculation of the concentration of the standard solution? Explain your answer in full.

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2 As shown in the diagram, there is an air bubble at the tip of a burette.2.1 How will the results be affected if the burettes are misread as shown in the diagram? Explain your answer in full.

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first reading

actual reading:41,40 mℓ

reading written in results:41,50 mℓ

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bottom of the burette with jet

air bubble jetnormally filled

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2.2 What effect does this have on the calculation of the concentration of the standard solution? Explain your answer in full.

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3. The indicator is a weak acid. How will the results be affected if 3 cm3 indicator is used instead of three drops? Explain your answer in full.

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PART 2You are provided with approximately 0,5 g of eggshells without any membrane on it (obtained from boiled eggs), hydrochloric acid with a concentration of 1 mol⋅dm-3, distilled water, phenolphthalein and sodium hydroxide with a standard concentration calculated for Part 1. Plan and conduct an experiment to determine the concentration of CaCO3 in the eggshell.

Aim: To calculate the percentage of calcium carbonate in eggshells.

Apparatus:

• Pestle and mortar• Watch glass• Electronic scale• Erlenmeyer flask• Wash bottle• 25 mℓ pipette• Glass stirring rod• Burette• Retort stand• Funnel• White tile

Chemicals:

• 0,5 g dry, clean eggshells• A standard sodium hydroxide solution of about 0,2 mol⋅dm-3 (As prepared for part 1.)• 1 mol⋅dm-3 hydrochloric acid• Distilled water• Phenolphthalein

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Method:

1. Using a pestle and mortar, grind the eggshells into a powder.2. Determine eggshell mass accurately.3. Place the eggshell powder in a 250 mℓ Erlenmeyer flask. Use distilled water to wash all the eggshell powder into the flask.4. Using a pipette, pour exactly 25 mℓ of 1,0 mol⋅dm-3 hydrochloric acid onto the eggshell and stir with a glass rod until no more bubbles forms. The hydrochloric acid is in excess.5. Clamp the burette upright in a retort stand and fill it with the standardised NaOH solution.6. Add 3 drops of phenolphthalein to the contents of the Erlenmeyer flask.7. Titrate the excess hydrochloric acid in the Erlenmeyer flask with the standard NaOH solution in the burette.8. Record the readings.9. Calculate the percentage of calcium carbonate in the eggshell.

Observations:

Record all qualitative observations.

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Results:

Mass of eggshelsl: ...............................................................................

Volume of 1,0 mol⋅dm-3 hydrochloric acid: ...............................................................................

Concentration of the standard NaOH solution (from Part 1): ...............................................................................

Volume NaOH titrated: ...............................................................................

Balanced reaction equations:Reaction 1: CaCO3(s) + HCℓ(aq) →

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Reaction 2: NaOH(aq) + HCℓ(aq) →

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Calculations:

Total number of mol of HCℓ added to the eggshells:

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Number of mol of NaOH required to neutralise excess acid:

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Number of mol of HCℓ in excess:

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Number of mol of HCℓ that reacted with CaCO3:

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Number of mol of CaCO3 in the eggshell:

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Mass of CaCO3 in the eggshell:

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% CaCO3 in the eggshell:

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Conclusion:

What is the percentage of CaCO3 in the eggshell?

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Questions:

1. Why would it not affect the results if the eggshell powder is washed into the Erlenmeyer flask with distilled water?

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2. Calculate the percentage accuracy of your results if 94% of eggshells consist of calcium carbonate.

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Experiment 22 Date: ........................................

Aim: To accurately determine the percentage of ethanoic acid per mass in commercial vinegar by titration against a standard sodium hydroxide solution.