pKa. When constant successive portions of Sodium Hydroxide are added to Acetic Acid; how do the changing amounts of Sodium hydroxide mixed with Acetic Acid in the conical flask affect the pKa of Acetic Acid?

Hypothesis:

In this experiment, the amount of Sodium Hydroxide released from the burette will never change. What changes, however, is how much base is in the conical flask. To begin with there will be no base but when 1cm3 of Sodium Hydroxide is added then there is 1mL of Sodium Hydroxide in the conical flask. Then when another 1 cm3 of Sodium Hydroxide is added there will be 2mL of Sodium Hydroxide in the conical flask. So what is changed is the amount of base in the conical flask and it is this amount of base that changes the pKa.

Thus, I hypothesize that as the amount of Sodium Hydroxide increases, pH will gradually increase until it reaches the end point where there will be a sudden increase. Afterwards, the amount of base will overtake the Acetic acid and this would result in a plateau in the pH curve.

Aspect 2

Defining Independent and Dependent Variables

Table 1: Dependent and Independent Variables

Controlling Variables

Table 2: Variables and Methods of Control

Aspect 3

Materials and Equipment List (enough for 1 titration including the rough titration)

• 1 x 50 +/- 0.05 cm3 Burette
•  1 x Burette Stand and clamp
• 1 x funnel
• 1 x 100 +/-0.050 cm3 Conical flask
• GLX pH probe
•  1L Distilled Water
• 1 x 20 +/- 0.020 cm3 Mohr Pipette
• 50 cm3 of Acetic Acid  concentration of approximately 0.1mol dm -3
• 100 cm3 Sodium Hydroxide solution 0. 1mol dm -3
• Safety Glasses
• White Paper
• Graph Paper
• Pen
• Pencil, for drawing the Graph
• Rubber Gloves (in case of glass breakage)
• 1 x White tile
• 1 x Standard Bulb
• 50 cm3 bottle of Phenolphthalein indicator (only 4 drops are required)
• Paper towels (for cleaning)

                

Figure 2: Conical Flask                      Figure 3: Burette                             Figure 4: Mohr Pipette

These pictures were all obtained from  

General Method

1. Before commencing this experiment it was made sure that all involved in the experiment were wearing the safety goggles and a lab coat to avoid risk of injury (refer to table 3)
2. The Titration was set up as follows, with the clamp holding the burette and the funnel at the top of the burette. The conical flask should be placed on a white tile underneath the burette. The burette should be clamped so that its tip is within the conical flask but above the surface of the solution.

                Figure 5: Titration set-up

3. The burette was then rinsed with distilled water to ensure that it is clean and to avoid errors
4. Usually experimenters cannot reach the top of the burette when its placed on a lab bench, so as a result, the burette and the burette stand were placed on the ground and Sodium Hydroxide was carefully poured to the first digit on the top of the burette (usually 0) via the funnel. The initial reading of the burette was then recorded in the results table, refer to table 3. Afterwards, the burette was returned to its original position on the top of the bench
5. The Mohr pipette was then checked for chips and cracks and was rinsed several times to ensure accurate volume measurements
6. Afterwards, the Mohr pipette was used to collect 20 cm3 of acid (Please refer to the ‘Using Mohr pipette method’) and the acid was then poured into the conical flask
7. Immediately following, the GLX pH probe, refer to GLX probe method, was adjusted and the head was placed in the acid, the pH of the initial acid was recorded in the results table, refer to table 3
8. 2 drops of Phenolphthalein was then added to the acid. The Phenolphthalein is an indicator which means it turns pink when base is added, an indication of endpoint would be that the liquid in the conical flask turns pink
9. To begin with, a rough Titration was be done to estimate the endpoint
10. In a rough Titration the tap was opened, and the base was simultaneously released into the conical flask until the liquid turned pink in which the tap was closed. When the liquid turned pink the final pH was recorded in the results table. Also record the final reading of the burette. The Rough Titration is only an indication and so should not be used in data analysis.
11. After the Rough Titration, the solution in the conical flask was discarded in the sink. Since the products were neutralized solutions of common salts they can safely be disposed of down the sink.
12. The conical flask was then washed with distilled water and 20 cm3 of Acetic acid was then poured into it via the Mohr pipette. And immediately following this, the GLX pH probe was placed inside the acid. Sodium Hydroxide was also then poured into the burette, using the method discussed in point 4, to top it up
13. The titration was then initiated, and successive portions of 1 cm3 of Sodium Hydroxide were added to the conical flask. After each addition, the burette and pH readings were obtained and recorded. If the person doing the experiment is right handed, then their right hand should be used to open and close the tap to allow Sodium Hydroxide to drip out and their left hand should be used to swirl the solutions in the conical flask.  Swirling is important as the pH may drift until a completely homogenous solution is achieved. The same principle applies to left handers, except they would of course use their left hand instead of their right hand to release the NaOH.
14. The Titration was continued until the pH reached 12, the final burette reading was also recorded
15. This titration process (steps 11-14) was repeated another two times, allowing more reliable results.
16. After the Titration the benches were wiped down using paper towels, the solution was discarded in the sink and the glassware was placed in the designated container. Hands were washed before leaving the laboratory.
17. By the results gathered, a Titration curve can be drawn. The pH would simple be on the y axis while the volume of the base would be on the x axis. The pKa can be determined by finding the pH halfway to the endpoint of the Titration curve.

GLX pH probe method:

1. The probe was first turned on and then pH probe section was plugged in at the top.
2. The pH probe section was then placed into the Acetic acid
3. Automatically, a pH reading appeared on the screen
4. When enough data was collected the recorded was stopped and the probe was unplugged
5. The GLX pH probe was then plugged into a laptop to export the data to the computer’s hard drive

Using Mohr Pipette method:

1. The standard bulb was attached to the end of the pipette by carefully fitting the bulb at the end of the pipette
2. The pipette tip was then placed below the surface of the liquid and the bulb was squeezed to draw the liquid up.
3. The volume of the Acetic acid in the pipette was determined by reading the meniscus of the pipette
4. The pipette was then held above the conical flask, and the bulb was released to release all the liquid into the conical flask

Table 3: The results table

Note: The purple shaded region is the rough Titration. The volume of Sodium Hydroxide added each time will always be 1 cm3 but the Amount of base in the conical flask will change. Also, the patterns in this table should be continued until the Amount of base in the conical flask is at least 30 cm3. The pH will be recorded via the GLX data probe.

Safety Precautions

BIBLIOGRAPHY

• "Volumetric analysis." 4 Feb 2009 <http://www.uwplatt.edu/chemep/chem/chemscape/LABDOCS/CATOFP/measurea/volume/pipet/pipet4.htm>.

• "Acid-base titrations." Wikipedia. 4 Feb 2009 <http://en.wikipedia.org/wiki/Acid-base_titration>.

• "pKa and LogP Measurements." 4 Feb 2009 <http://www.raell.demon.co.uk/chem/logp/logppka.htm>.

• "pKa of a weak acid." 4 Feb 2009 <http://209.85.173.132/search?q=cache:E6yRz3RiHlwJ:www.profpaz.com/Files/chem102/Exp_10.pdf+%22volume+at+equivalence+point%22&hl=en&ct=clnk&cd=1&gl=au>.

• Neuss, Geoffrey. Chemistry . London: Oxford, 2007.

• Neuss, Geoffrey. Chemistry For the IB diploma. London: Oxford, 2007.

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