ANSWERS TO QUESTIONS:
1. What are the significant regions in the titration curve? Relate the characteristics of each region to the pH results obtained.
- There are four significant regions in each titration curve, namely the initial, pre-equivalence, equivalence, and post-equivalence points. These points are named according to its position relative to the equivalence point.
The equivalence point signifies the volume of titrant at which the solution becomes neutral. This is represented in the graph as the drastic change in pH, first with a sudden increase and a sudden decrease of pH.
At the initial point, no titrant has been added yet and the solution’s pH equates to the number of H+ ions originally present in the analyte.
At the
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Using this method, the amount of titrant used for weak acids would be the same as that used for strong acids since it is the number of protons or H+ ions, measured through the pH, which determines the equivalence point. With normal titrations of strong acids and bases, since H+ completely dissociates, similar concentrations with a weak acid and strong base titration would not be the same since weak acids do not dissociate completely. Another advantage of potentiometric titration is the fact that indicators are not used, therefore human error due to failure of recognition of color change may be avoided.
5. Discuss possible reasons behind the discrepancies (if any) in the experimental and theoretical pKa values.
- Human errors may have affected the results, such as miscalculations, overtitrations and failure of determination of the right pH value since the pH meter tends to fluctuate. Another source of error could be from the pH meter apparatus itself, particularly the electrodes since the classroom’s conditions are not perfect. The failure to rinse the electrodes may result to drastic changes as well. Another error may result from the failure in accuracy of the analytical balance since it has been used for several years prior.
6. What are the possible sources of errors and their effect on the calculated parameters? Rationalize.
- One of the members of the group accidentally touched the magnetic stirrer therefore contaminating the whole set-up. Since it cannot be
We know that that the end point of the titration is reached when, after drop after careful drop of NaOH, the solution in the flask retains its pale pink color while swirling for about 30
1. To titrate a hydrochloric acid solution of “unknown” concentration with standardized 0.5M sodium hydroxide.
9) Filter the ppt formed as result of step 7 from the newly acidic solution.
pH was recorded every time 1.00 mL of NaOH was added to beaker. When the amount of NaOH added to the beaker was about 5.00 mL away from the expected end point, NaOH was added very slowly. Approximately 0.20 mL of NaOH was added until the pH made a jump. The pH was recorded until it reached ~12. This was repeated two more times. The pKa of each trial are determined using the graphs made on excel.
A buffer is a solution that resists changes in pH when H+, OH-, or H20 is added. By using standard lab equipment, a lab pro diagnostic tool, and acidic and basic solutions, the pH can be found. By recording the pH while adding a base or an acid gradually to a buffer solution you can find the capacity of each buffer to resist drastic changes in pH. The best buffers will keep a solution from becoming either too acidic or basic with the addition of a strong base or acid.
Question: The equivalence points of the two titration curves were not in the same pH range. Explain. (Why was the pH at the equivalence point different for the two different acids?)
To improve the results from the experiment buffer solutions that were not whole pHs could have been used e.g. pH 4.5, 5.5 etc. This would have provided more reliable results as a wider range of results would have been produced. Using pHs with decimals would also help to more accurately determine the optimum pH as the optimum may have been above or below the pH stated in the hypothesis; 8. In this experiment however the optimum is taken at 8 because the graph does not rise again.
Table 2: Consists of color extract taken from a red cabbage for a natural indicator. The pH reading that was measured by using the pH meter and the result of the pH reading to determine whether the solution was acidic or basic.
Figure 1: Amount of O2 gas curves to the time at which it was measured according to low, medium, and high pH.
To begin, three sets ofabout 0.3000g of KHP are weighed out on an analytical balance. Put the three sets of KHP into three separate, labeled flasks. All three sets of the KHP is then dissolved with approximately 50mL of deionized water. Next, a buret is used to start the actual titration. Buret is initially filled to 0.00mL mark with the NaOH solution, this is recorded as initial volume. Next, add 2-3 drops of phenolphthalein indicator into each of the three flasks containing KHP. A magnetic stir bar is then added to the first flask, and placed above a stir plate. Everything is positioned under the buret. Stirrer is put on medium speed and the titration can start. Slowly release the NaOH into the KHP flask. As the end point is reached, a pink color will be seen in the flask. When the lightest pink possible remains in the solution for more than 30 seconds titration is complete. The final volume is recorded, and the same steps are taken for the other two sets of KHP solution. Finally, blank titration is completed to determine deviation.
Within an acid-base titration the titration curve resembles the strengths of the corresponding acids and bases. A strong acid will correspond with a weak conjugate base, and a weak conjugate acid will correspond with a strong base. This is based on the Bronsted-Lowry model. The weak acid will donate protons to the hydroxide ion. Weak acids will have a low Ka value, the Ka value is the tendency of the acid to dissociate:
Using Graph 1: The Volume of Titrant Added in order to reach the Endpoint and the Corresponding pH Values, observe the vertical line of each titration and see the points in which the horizontal lines intersect it. These points give the
First, three titration curves and three second derivative curves were created to determine the average pH at the half-equivalence point from the acetic acid titrations. Titration curves were used as visuals to portray buffer capacity. The graphs and a table, Table 1, that showcased the values collected were created and included below. The flat region, the middle part, of Figures 1, 2 and 3, showed the zone at which the addition of a base or acid did not cause changes in pH. Once surpassed, the pH increased rapidly when a small amount of base, NaOH, was added to the buffer solution. Using the figures below and
2. Following solutions are added to the tubes and the pH of each tube is determined:
Write a discussion of error summarizing 2 sources of error, why are your results and the other group different?