February 21, 2013
With the given volume measurement of KHP and NaOH, the students experimented to calculate the mass of unknown acid and its equivalence point using a method called titration and titration curve. The known volume of an acid solution would be titrated by slowly adding drops of solution into NaOH, and the volume of base needed to react with the acid is measured throughout. By using those data, the students are able to find what they were trying to calculate. On this lab, the students found the unknown acid, 4.36 x 10-5, and the equivalence point of 8.4, as well as the value of pH. Purpose:
The purpose of the experiment was to utilize the techniques of titrations to measure the concentration of an acid or base in solution, to calculate molar mass of an unknown acid or base, and to determine the equilibrium constant of a weak acid or weak base. Materials:
pH sensor or pH meter
Ring stand and buret clamp
Part 1: Standardization of a Sodium Hydroxide Solution
1. Obtain a sample of pre-dried potassium hydrogen phthalate (KHP) 2. Weigh 0.4-0.6 grams of KHP, then record in Data Table 1. 3. Transfer the KHP into the Erlenmeyer flask using the funnel, then use the spray bottle to get any remaining solid into the flask. 4. Add 40 mL of water to the flask and whirl until thoroughly dissolved. 5. Obtain 75 mL of sodium hydroxide (NaOH) solution.
6. Clean a 50-mL buret, then rinse it with three small portions of NaOH. 7. Fill the buret to above the zero mark with the NaOH solution. 8. Open the buret stopcock to allow any air bubbles to escape from the tip. Close the stopcock when the liquid level is between 0 and 10- mL 9. Measure the precise volume of the solution in the buret and record this value in Data table 1 as the “initial” volume. 10. Position the buret over the Erlenmeyer flask so that the tip of the buret is within the flask but at least 2 cm above the liquid surface. 11. Add three drops of phenolphthalein solution to the KHP solution in the flask. 12. Begin the titration by adding 1.0 mL of the NaOH to the Erlenmeyer flask. Close the buret stopcock and swirl the flask to mix the contents. 13. Repeat step 12 until 15 mL of the NaOH solution have been added to the flask. Be sure to continuously swirl the flask. 14. Reduce the incremental volumes of NaOH solution to .05 mL until the pink color starts to persist for 15 seconds, while constantly swirling. 15. Measure the volume of NaOH solution remaining in the buret, estimating to the nearest .01 mL and record this value as the “final volume” in Data table 1 16. Repeat the standardization titration two more times. Rinse the flask thoroughly between each trial with water. Part 2: Determination of the Equivalent Mass of an Unknown Acid 1. Weigh about 0.3-0.4 g of a sample of the unknown acid and record the precise mass in Data table 2. 2. Dissolve the unknown acid in 40 mL of water and titrate to the phenolphthalein endpoint as above in steps 5-16. 3. Record the initial and final volumes of NaOH in Data table 2. 4. Repeat once more, choose a mass for the second sample so that the volume of NaOH needed will be about 45 mL when using the 50 mL buret. Part 3: Determination of the pKa of the Unknown Acid
1. Set up a pH meter and electrode. Calibrate the pH meter isong a pH 7.0 buffer. Rinse the electrode well with water. 2. Weigh a sample of unknown acid that will require approximately 20 mL of titrant. 3. Dissolve the acid in approximately 100 mL water in a 250-mL beaker. 4. Fill the buret with the standardized NaOH solution used in part 1. Record initial volume as the “initial buret reading” in Data table 3. 5. Stir the beaker containing the unknown acid solution. Submerge the pH electrode into the acid solution. 6. When the pH reading has stabilized, record the...