PART 2
Aim:
- How does the addition of small amounts of strong acid and strong base affect the buffer set up in part 1? And what is the effect of diluting the solution on its buffering capacity? Does it act as a better buffer when small amounts of acid are added to it or small amounts of base?
Hypothesis: The introduction of small amounts of acid or alkali to the buffer solution should have little effect on the pH of the solution due to the nature of the buffer solution. Also, the buffer should act better when small amounts of acid are added to it as compared to the addition of a base. This is because the buffer is weakly acidic, hence it will have a greater tendency to become basic as compared to becoming more acidic.
Apparatus: 50 cm3 Burette, 250 mL beakers, 25 cm3 pipette, 0.1 M HCl, 0.1 M NaOH
Method:
- Set up two burettes one filled with 0.1M HCl the other with 0.1M NaOH.
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Then, using a pipette measure out exactly 25cm3 of buffer solution and place it in a small beaker. Repeat this so that there are two small beakers each containing 25cm3 of buffer solution.
- Add HCl to one of the beakers, and NaOH to the other, using two separate burettes.
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First drop 0.1 cm3, then 1cm3 up to 5cm3 measuring the pH after each step and recording it in a table.
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Then make two solutions, the first one should be 1/10 dilution, and for this 10cm3 of buffer solution is measured and then 90cm3 of distilled water measured out using a burette should be added to it.
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The second will be a 1/100 dilution; for this 2cm3 of buffer solution is measured and 198cm3 of distilled water should be added to it.
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Then, using a pipette measure out exactly 25cm3 of dilute buffer solution and place it in a small beaker. Repeat this so that there are four small beakers each containing 25cm3 of buffer solution.
- Then, add HCl to one of the beakers, and NaOH to the other for each dilute buffer solution, using two separate beakers.
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First drop 0.1 cm3, then 1cm3 up to 5cm3 measuring the pH after each step and recording it in a table. Do these for the solutions in all the four beakers.
Data Collection:
Data Processing:
1)
2)
3)
4)
Error Of Uncertainty:
% Error of burette + % Error in pipette + % Error of pH meter
% Error = 0.02/50 + 0.12/25 + 0.4/4.8
= ± 8.85 %
Conclusion and Evaluation:
From the very beginning there was an Error in the value of the Initial pH. The value was approximately 4.8. Ideally, the initial pH should have been 5.2, since the volume of conjugate base added was three times that of the weak acid. Along with the random error, the main reason for this error in the pH meter was the systematic error in the pH meter. There was a great deal of fluctuation in the pH meter hence recordings were not accurate. Using better equipment such as pH probes, which have minimal systematic error, could solve this problem.
Although, there was such a huge error, our main hypothesis can be proved. The trends give us a fair idea of the buffer capacity of the buffer solution. From graphs 1 and 2, we can clearly see, using the curve fit, that there is a very small change in the pH when small amounts of acid and base are added to it. Furthermore, the buffer solution acts better when small amount of acid are added to it, as compared to the addition of base. This is observed from all the 4 graphs. The curve is less steep when 0.5 M HCl solution and 1/5th diluted HCl solution is added to it, as compared to when 0.5 M NaOH and 1/5th diluted NaOH is added to it.
Another reason for the error in the buffer capacity is that 0.5 M HCl and NaOH is added. If we had used 0.1 M HCl and NaOH, then fewer moles of the acid and base would have been added, thus allowing the buffer to act for a longer time.
