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Introduction

Experiment E5 Preparation and Properties of a buffer Date: 23.3.2011 Name: Leung She Ting Cherry Class: 6A 20 Objective To prepare a buffer solution and observe the properties of a buffer Introduction Buffer is a solution that can resist dramatic pH change when a small amount of acid or alkali is added to it. It is made up of equal amount of a weak acid/alkali and its conjugate base/acid. The working principle of buffer can be demonstrated as below: Ethanoic acid is a weak acid that undergoes slight dissociation, CH3COOH (aq)+H2O(l) CH3CHCOO-(aq) + H3O+(aq) where the equilibrium position mainly lies on the left side. At the same time, sodium ethanoate undergoes complete dissociation, CH3COONa (aq) CH3COO- (aq) + Na+ (aq) in which its equilibrium position lies mainly on the right side. When a small amount of acid is added to the solution, the addition of H3O+(aq) will shift the equilibrium of CH3COOH (aq)+H2O(l) CH3CHCOO-(aq) + H3O+(aq) to the left. The increase of [CH3COOH] does not cause a significant change in pH, thus the pH of the solution is kept almost constant. On the other hand, if a small amount of base is added to the solution, CH3COOH(aq) ...read more.

Middle

4. 4 cm3 of HCl was added to the beaker. The reaction mixture was stirred with a clean glass rod. 5. The pH value was measured with the pH meter. 6. The above 5 steps were repeated using 0.1M NaOH instead. 7. The same procedures were conducted with solution B. Testing of effect of dilution on pH 1. 5 cm3 of Solution A was measured with a clean measuring cylinder and poured into a 100 cm3 beaker. 2. 45 cm3 of distilled water was measured using clean measuring cylinder and poured into the beaker. 3. The diluted reaction mixture was stirred with a clean glass rod. 4. The pH of the reaction mixture was measured. 5. The above procedures were repeated using solution B. Chemicals used 0.1M sodium hydroxide 0.1M ethanoic acid 0.1M hydrochloric acid Apparatus Beaker 50 mL Measuring cylinder 10 mL Measuring cylinder pH meter Glass rod Beaker pH meter Measuring cylinder Result table pH value Solution A Solution B Original solution 4.5 4.5 After addition of 1 cm3 of 0.1M HCl 4.5 2.3 After addition of a total 5 cm3 of 0.1M HCl 3.9 1.8 After addition of 1 cm3 of 0.1M NaOH 4.8 11.4 ...read more.

Conclusion

This shows solution A, a buffer, can resist the pH change upon addition of acid, base or water. The calculated and measured values of pH vary within a range of 0.1 to 0.2. One of the possible errors is that the experiment was not performed at room temperature, i.e. 25?/298K. The temperature in the laboratory was about 20.9?. The value of dissociation constant, Ka =1.74�10-5 provided is not applicable in the experiment. The actual dissociation of hydroxonium ions was less than the standard one. Measuring cylinders were used in the experiment. There might be inaccuracy in the reading on the cylinder. Thus, the solutions prepared might not be of known concentration or the volume of acid or base added was not exactly 1/5 cm3. Pipette might be used to transfer the solution instead as it has a higher accuracy. pH meter was used to measure the pH values of solutions. However, the accuracy and precision of pH meter varies. If the precision of the pH meter is not high enough, a slight change in pH value may not be shown. An inaccurate pH meter also gives invalid pH values. A pH meter with higher precision may be used and calibration of pH meter helps to improve the accuracy. ?? ?? ?? ?? ...read more.

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