Calculate the approximate volume of HCl you will use in the back-titration step if the mass of ASA is 500 mg per tablet. (The procedure is as described in the experiment below using 0.10 M HCl and 0.10 M NaOH).

The equation is C9H7O4+2OH----C7H5O3+CH3CO2+H2O

Preparation of Carbonate Free NaOH

Prepare a solution of sodium hydroxide as follows:

1. Boil 1.5 L of distilled water and allow it to cool.

2. Roughly weigh 5 g of NaOH pellets (MW 39.997 g/mol) on a watch glass.

Sodium Hydroxide is

very caustic. Ensure that you clean up spills immediately.

3. Rinse off surface Na2CO3 with a small quantity of distilled water from a wash bottle. Discard the

   rinsings. Do not allow the pellets to sit in the liquid, or you will dissolve too much of the NaOH,

   and your solution will be too weak.

4. Dissolve the pellets by adding them to 5 mL boiled distilled water in a small beaker. This will give

   a 50% solution of NaOH in which Na2CO3 is practically insoluble. If some undissolved Na2CO3 remains, allow this to settle and carefully decant 4 to 5 mL of the liquid or remove it with a pipette into a little distilled water that has been freshly boiled and cooled. Avoid transferring the carbonate in this step.

5. Transfer the solution to a 1-litre plastic storage bottle and add approximately 995 mL of the boiled water (enough to almost fill the bottle), stopper the bottle well and mix thoroughly by inversion. The solution will now be about 0.1M.

6. Always store your solution in a plastic bottle. Before storage, squeeze the bottle slightly as you put on the cap to expel as much air as possible. This will minimize the amount of CO2 that is absorbed into the solution.

Standardization of NaOH using Potassium Hydrogen Phthalate

1. Rinse your burette with about 5 mL of the carbonate-free 0.1 M NaOH solution prepared above. Ensure that the burette tip is rinsed as well. Discard the solution.

2. Fill the burette with NaOH solution and remove any air bubbles from the tip or sides of the burette. Adjust the solution level to approximately 0 mL.

3. Remove the drop at the end of the burette by touching it to the side of a waste beaker.

4. Read the starting volume accurately. The burette should be read to a precision of 1/5 of the

   smallest division, i.e. to a precision of 0.02 mL.

5. Set up a table in your lab notebook.

6. Using a weighing boat or weighing paper, weigh with precision a quantity of 0.7 to 0.9 g of pre- dried potassium hydrogen phthalate (stored in the desiccator) to the nearest 0.1 mg and rinse into an Erlenmeyer flask.

Mass KHphthalate /g	Moles KHphthalate /mol	Vol. NaOH /mL	Conc. NaOH /M

20

7. Calculate your rough titre (volume of NaOH that must be added to reach the equivalence point) based on the mass of the standard.

8. Add to each sample 50 to 75 mL of boiled distilled water. Stopper the flasks and swirl to dissolve the solid.

9. Add 3 drops of phenolphthalein

10. Add 75% of the calculated volume of the NaOH (from step 7) from the burette into the conical

    flask with rapid agitation. Rinse the sides of the flask with distilled water.

11. Add the NaOH dropwise, mixing well between each drop until a faint pink colour is seen, which persists for 30 seconds. (This colour may fade after a few minutes owing to the uptake of carbon dioxide. If it doesnt fade within a few minutes, then you have over titrated your solution). If you have some difficulty seeing the pale pink colour of the indicator, a doubled quantity (6 drops) may be employed without a serious error being introduced. Ensure that any drops on the end of the

    burette tip are added to the solution, and accurately read the final volume on the burette.

12. Repeat this standardization 2 more times using new samples of potassium hydrogen phthalate for

    each trial.

13. A blank titration must be carried out, and any blank titre should be subtracted from all titrations.

    This is done by titrating 5 mL of 0.5 M sodium chloride with the NaOH solution. Treat the solution

    as above. (It will only take a drop or two of the NaOH solution, if any at all.)

14. Calculate the molarity of the NaOH solution by taking the mean of your three trials. The deviation of these values from the average should not exceed 0.5% relative error if you are to proceed to the

    next part of the experiment.

Preparation of 0.1 M Hydrochloric Acid

1. Add 8.6 mL of concentrated hydrochloric acid (11.6 M) to 500 mL (use a measuring cylinder) of tap water in a glass storage bottle and makeup to 1 litre by adding approximately 490 mL water. Concentrated acid should always be added to water, never the other way around.

2. Mix well by inverting the sealed bottle several times.

Standardization of 0.1 M Hydrochloric Acid

1. Rinse the 25 mL pipette 3 times with small portions of HCl.

2. Pipette 25 mL of the hydrochloric acid into an Erlenmeyer flask.

3. Add 2 -3 drops of phenolphthalein to the solution in the Erlenmeyer flask and titrate with

   standardized 0.1 M NaOH as described above.

4. Repeat the titration until 3 titrations differ by no more than 0.5%.

5. Calculate the concentration of the HCl.

Preparation of Acetyl Salicylate Solution

1. Weigh with precision 3 aspirin tablets and place each tablet in a separate 100 mL volumetric flask. (The flasks must be clean and dry. Remember not to dry the flasks in an oven.)

2. Label each flask clearly.

3. Add approximately 75 mL of 0.1 M NaOH to each flask and shake well until the tablet is dissolved.

   A small quantity of white residue may not dissolve.

4. Fill the flask to the 100 mL calibration mark with 0.1 M NaOH.

5. Shake well to mix contents.

Back titration of the excess NaOH

1. Empty the burette and wash well with tap water, then rinse with distilled water.

2. Rinse the burette with HCl before filling.

3. Use the same procedure as previously used with the NaOH.

4. Pipette 25 mL of the acetylsalicylate solution into an Erlenmeyer flask. If there is any residue on

   the bottom of the flask, be careful not to disturb it or suck any of it into the pipette.

5. Add 2 - 3 drops of phenolphthalein and titrate with the hydrochloric acid until a faint pink colour

   persists.

6. Repeat the titration 2 more times. Your results should agree within 0.5%.

7. Calculate the initial quantity of NaOH (in 25 mL), the amount of NaOH in excess and determine

   the quantity of acetylsalicylic acid in each tablet.

8. Calculate the ASA as a %wt.

Analysis of aspirin powder

1. Several aspirin tablets have been crushed and ground into a homogenous mixture. You will be given 1 portion of this powder to analyze for the concentration of aspirin.

2. Precisely weigh an amount of aspirin powder roughly equal to the weight of an aspirin tablet.

3. Analyze the ASA as described above.

4. Calculate the amount of ASA as a %wt concentration.