Experiment: Analysing molar mass of Aspirin

C6H5(OCOCH3)COOH        +  NaOH        ------>                  C6H5(OCOCH3)COO+Na-           +        H2O

                     (acid)                 (base)                                       (salt)

The theoretical mass of aspirin can be calculated by considering the relative atomic mass of its elements. The molecular weight calculation is as follows:

(12.0107*9) + (1.00794*8) + (15.9994*4)=180.15742 g/mol

Apparatus and Materials

Procedure

1. Place an aspirin tablet in about 20ml of water and let stand for a few minutes. The tablet will break apart as it absorbs water and swells
2. Add 20ml of ethyl alcohol / ethanol to dissolve aspirin
3. Add 3-4 drops of phenolphthalein indicator to the solution
4. Rinse out a burette twice with water, then drain 10ml of 0.1M NaOH
5. Fill the burette with NaOH and record the initial volume
6. Titrate with 0.1M NaOH to the end point ( the light pink colour) observed
7. Record the final volume of the NaOH
8. Repeat the experiment, but instead of phenolphthalein, use bromothymol blue as indicator. It is not necessary to clean out your burette of NaOH before you start the second experiment. Simply refill your burette and then start again.

Data Collection and Processing

Raw data

Calculation

1. Experiment A

C6H5(OCOCH3)COOH        +       NaOH           ------>      C6H5(OCOCH3)COO+Na-           +        H2O

Average volume of NaOH used=

 = 16.1 ml

No. of mol of NaOH=

= 0.00161 mol

No. of mol of C6H5(OCOCH3)COOH, x = 0.00161*1 = 0.00161 mol

Mass of one tablet of aspirin, C6H5(OCOCH3)COOH= 300 mg or 0.3 g

Molar mass of C6H5(OCOCH3)COOH =

=

 = 186.33541 gmol-1

        Percentage error=

= 3.429%

2. Experiment B

   C6H5(OCOCH3)COOH           +       NaOH              ------>         C6H5(OCOCH3)COO+Na-           +        H2O

        Average volume of NaOH used=

 = 15.5 ml

        No. of mol of NaOH=

= 0.00155 mol

        No. of mol of C6H5(OCOCH3)COOH, x = 0.00155*1 = 0.00155 mol

        Mass of one tablet of aspirin, C6H5(OCOCH3)COOH= 300 mg or 0.3 g

        Molar mass of C6H5(OCOCH3)COOH =

=

 = 193.54838 gmol-1

        Percentage error=

= 7.433%

Conclusion

The experimental molar mass of Aspirin, C6H5(OCOCH3)COOH is close to the theoretical value. The difference between the experimental value and the accepted value is not really big. However, there are difference between the experimental molar mass of aspirin when using phenolphthalein and bromothymol blue as indicator. The obtained molar mass of aspirin when using phenolphthalein indicator is smaller than using bromothymol blue as indicator. The difference with the theoretical value is also less. Due to this difference, the percentage error when using phenolphthalein is smaller than bromothymol blue. Therefore, phenolphthalein is a better indicator in this acid-base titration reaction. This is because the final pH or the endpoint of titration lies in between pH range of phenolphthalein that is 8.2-10.0 and not in bromothymol blue which is 6.0-7.6 . This is the reason why phenolphthalein indicator is able to produce more accurate result than bromothymol blue.

Evaluation

        There are some weaknesses that were detected throughout the experiment. Firstly, the volume of indicator are not fix in this experiment. Using drops is not really suitable as it may distort the volume of indicator in this experiment. The difference may not really big but it still can affect the final result.

        Secondly, the mixture of aspirin, C6H5(OCOCH3)COOH and sodium hydroxide, NaOH is not stirred during the titration. This may produce uneven and not uniform result as the NaOH is not totally mix with the aspirin. The neutralization process may only occur on the surface of the aspirin solution.

        Having said that, this experiment is successfully analyzing the molar mass of aspirin. The experimental value is close to the theoretical value.

Further improvement

In order to improve the results of the future experiments, several ways can be implemented. Firstly, the volume of indicator use should be fixed instead of using drops. By using this way, the level of uncertainty of the final results can be reduced. Besides that, the mixture of aspirin solution and NaOH need to be stirred throughout the titration process. More uniform results can be obtained by implementing this method.