Experiment 3 ACID - BASE TITRATION – POTASSIUM HYDROGEN PHTHALATE INTRODUCTION Potassium hydrogen phthalate or KHP (molecular weight = 204.23 g/mole) is a weak organic acid whose structure is as follows: Since KHP contains only a single "acidic" or ionizable hydrogen, it is referred to as a monoprotic acid. Therefore, in its reaction with a base, such as hydroxide (OH-), each KHP can lose only one hydrogen ion (H+): In this experiment, the unknown will consist of a white, water-soluble powder which will be titrated with NaOH in order to determine the % KHP in it. The experiment consists of three principal steps. First, the NaOH solution to be used as titrant is prepared by dilution of a stock solution to approximately 0.1 M. Second, the exact concentration of the NaOH solution is determined by titrating it against a known amount of a pure KHP standard. Last, the recently standardized NaOH solution is used to titrate an unknown sample. PROCEDURE 1. Preparation of 0.1 M NaOH. Using a graduated cylinder, measure approximately 4 mL of the provided 6 M NaOH solution, transfer to a 250 mL volumetric flask and dilute to the mark with deionized water. Remember to use care with NaOH solutions; they are strong bases which could damage eyes, skin, and clothing. 2. Standardization of NaOH against KHP. Weigh out (to 4 decimal places) 0.3 - 0.5 g of pure, dry primary standard KHP (provided in a desiccator) into a 250 mL Erlenmeyer flask and dissolve in approximately 50 mL of deionized water. Add 3-5 drops of phenolphthalein indicator and titrate with the prepared NaOH solution using a buret until a faint pink color appears that persists for 30 sec. As in Experiment 2, you should record the initial and final buret readings and the color should not be dark pink or purple at the endpoint. Because the faint pink color should fade away with time if you don’t over-titrate (Can you tell why?), you should not worry about color changes in the solution once you finish the titration. Repeat the titration at least two more times. Calculate the average NaOH concentration from the three measurements and use this average NaOH value in subsequent calculations involving the unknown. Make sure you use the appropriate number of significant digits. 3. Analysis of the unknown. Obtain your unknown from the TA. Weigh out 0.8 - 1.0 g of the unknown (to 4 decimal places) into a 250 mL Erlenmeyer flask. Dissolve the unknown in about 50 mL of deionized water, add 3-5 drops of phenolphthalein, and titrate with the same NaOH solution that you previously standardized until the pink color of the endpoint is reached. Repeat at least two more times. Try to derive the equations needed to calculate the weight of KHP and the % KHP found in each trial. See the TA if you need help or if your calculation needs to be checked. Report the average % KHP found for your three trials along with the 95% confidence interval. Of course, apply the Q-test before calculating your average value. Clean-up: All NaOH solutions should be disposed of in the basic waste container. Excess KHP and unknown samples can be discarded in solid waste container. Light pink titrated samples can be poured down the sink with running water. Chem 207 Pre-Lab Questions for Experiment 3: Acid – Base Titration Potassium Hydrogen Phthalate Name __________________________________ TA and Section __________________________ 1. (3 points) What are the three principal steps in this experiment? 2. (2 points) Calculate the volume of 6.0 M titrant solution that you would need to prepare 250 mL of a 0.20 M NaOH solution. 3. 4. (4 points) A student pipets 35.00 mL of 0.0919 M HCl solution into a flask. a. How many moles of HCl are present? b. How many mL of 0.0919 M NaOH will be required to neutralize the HCl in part a? (4 points) A solution of KOH is prepared and is standardized against 0.5418 g of KHP. The phenolphthalein endpoint was reached by the addition of 25.91 mL of KOH. What is the molarity of the KOH solution? 5. (3 points) An unknown sample (0.6855 g) containing KHP is titrated to the equivalence point with 9.75 mL of 0.07892 M NaOH. a. How many moles of KHP are present? b. How many grams of KHP are present (molecular weight of KHP = 204.23 g/mole)? c. What is the percent KHP in the sample? 6. (4 points) Recalculate 5 assuming that the student accidentally added 4 drops (0.20 mL) too much NaOH. What is the % KHP? What is the percent error assuming that the % reported in problem 5 is correct? For TA use Grade __________________________ Chemistry 207 Report Sheet Experiment 3 Name ________________________ TA and Section ________________ Acid-Base Titration - KHP Date Performed ________________ Unknown Number ______________ NaOH STANDARDIZATION Trial 1 Trial 2 Trial 3 Weight of pure KHP used __________ __________ __________ Initial buret reading __________ __________ __________ Final buret reading __________ __________ __________ Volume NaOH used __________ __________ __________ Molarity NaOH __________ __________ __________ Calculations (show equations clearly): Q-test of Molarity NaOH (90%): Confidence Limit of Molarity NaOH (95% confidence level): Average M NaOH _____________ UNKNOWN TITRATION Trial 1 Trial 2 Trial 3 Weight of unknown used __________ __________ __________ Initial buret reading __________ __________ __________ Final buret reading __________ __________ __________ Volume NaOH used __________ __________ __________ Weight KHP __________ __________ __________ % KHP in unknown __________ __________ __________ Calculations (show equations clearly): Q-test of % KHP (90%): Confidence Limit of % KHP (95% confidence level): Average % KHP ± 95% C.L. ______________________ For TA use Date Received Unknown Grade
