P H at the UN Part III Lab PDF

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pH at the UN Part III Cover Sheet Iffath Islam Chemistry 106 Section 2 May 5, 2014 ; Spring 2014 Waqar Rizvi Group Member: Aryna Apanaska

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Iffath Islam May 5, 2014 Chemistry 106 pH at the UN Part III Observations: Case 1- After 9.00 mL of NaOH was added to the HCl, a tinge of green was observed in the solution. However, once 9.50 mL of NaOH was added, the solution turned blue. Case 2- After 9.00 mL of NaOH was added to acetic acid, a yellow-green color could be observed in the solution. Once 10.00 mL of NaOH was added, the solution turned a blue color. Case 3- After 8.00 mL of NaOH was added to the buffer solution, a yellow-green color was observed in the solution. Once 10.00 mL of NaOH was added, the solution turned blue. Post-Lab Questions: Case 1: 1. Graph 1A is more useful because it allows us to observe the change in pH of the solution after the addition of a strong base, NaOH. Graph 1B allows us to only see the change in concentration of the solution due to the addition of the NaOH. It is already known that through the addition of the NaOH, the concentration of the H+ ions will decrease. Therefore, Graph 1B is not as helpful. Graph 1A, on the other hand, allows us to see at what volume the pH of the solution changed and at what volume the solution reached its endpoint. 2. At the beginning of the plot, there was only HCl as it was pure. In the middle of the plot, when the solution neutralized, there was H2O in solution. At the end of the plot, there was excess base (NaOH). 3. From looking at the graph, the endpoint of the reaction occurred at around a pH of 7 and a volume around 9.5 mL. Near the endpoint, the titration should be performed slowly because any extra volume of base would cause the pH of the solution to skyrocket. This can be seen in the titration curve in the region that is almost vertical. Prior to the vertical increase, the pH increases very slowly and then almost instantly shoots up in a matter of a few mLs of base. 4. The numerical difference between pH = 1 and pH = 2 is that has a pH of 1 requires a concentration of 0.1 H+ ions. A pH of 2 requires a concentration of 0.01 H+ ions. Therefore, a pH of 1 is 10 times more acidic than a pH of 2. 5. There is a minus sign before the log because very rarely are the concentrations of the H+ ion above 1. By adding a negative sign before the log, we are able to obtain positive pH values. Case 2: 1. At the beginning of the reaction, the major species is the acetic acid. At the equivalence point, there was H2O in solution due to neutralization and NaOH

Islam 3 since acetic acid is a weak acid. At the end of the reaction, there was excess NaOH in solution. 2. The pH curve in case 2 is different from case 1 because of the initial pH. Since acetic acid is a weaker acid than HCl, the initial pH reading was higher. Furthermore, the equivalence point pH was also higher for case 2 than it was for case 1. Furthermore, the titration curve in case 1 began almost as a flat line. For case 2, there was a very steady increase in the pH after which the titration curve became almost flat. Overall, the S shape was maintained in the graph for case 2. Case 3: 1. At the beginning of the titration, there was only the acidic buffer. At the equivalence point, the buffer is still present as is the NaOH. At the end of titration, the major species is NaOH. 2. The pH curve for the buffer solution already began at a high pH which is one remarkable difference between case 1 and case 3. Additionally, the increase in pH was not very pronounced as it was in case 1. Both titration curves for case 1 and case 3 were an S shape. 3. The pH curve for case 2 and case 3 are similar because both solutions start at a higher pH. This signifies that they are weak acids. For case 3, it is known however, that the solution is a buffer and is composed of both acid and base. The increase in pH for both graphs was not tremendous although the increase in case 2 was greater than that of case 3. The S shape again is maintained in the titration curve. 4. The equivalence is important to chemist because it allows them to pinpoint the volume of base and/or acid required to successfully titrate the species and reach a neutral solution. Focus Question: Making titration curves are useful because they allow us to see the change in pH during titration. From titration curves, the equivalence point can be determined which allows us to see at what volume the solution is neutralized. Furthermore, by using a pH curve, the concentrations of the species at different points in the reaction can be calculated....