BUFFER PREPARATION AND TESTING Aims : To prepare a buffer solution of a specific. : To test the prepared buffer and compare with the theoretical value calculated. Introduction A buffer is a solution that resists a change in pH, when either small amounts of an acid or a base are added because it contains species in solution able to react with them according to the principles of equilibrium. In this experiment, the practical matter of preparing a buffer at a desired pH will be between acetic acid and sodium acetate. The Henderson-Hasselbach equation, which can be easily derived from equilibrium equations, is used to find the pH of a buffer solution. You will be required to make a buffer using acetic acid (HC2H3O2) (pKa = 4.756) and sodium acetate (NaC2H3O2), the sodium salt of the conjugate base. Note the relationship between the acid and its conjugate base in the equilibrium You are provided with 5 mL acetic acid solution of 0.3 mol.dm-3 in order to prepare a buffer of pH = 5 in total volume of 100mL. Work out the quantity of sodium acetate required to adjust pH of buffer to 5, (laboratory Instructor will approve). Material Calibrated pH meter, 0.3M Acetic acid, sodium acetate salt analytical balance, measuring pipette, two 100mL beakers magnetic stirrer and bar. Procedure: Part A. pH = 5.0 Buffer Solution 1. Transfer about 50 mL of distilled water into a clean 100 mL beaker. 2. Use a measuring pipette to add 5.0 mL of 0.40 M acetic acid to the same beaker. 3. Weigh accurately an empty small beaker and record its mass to all the sig figs possible , add about 0.3 g sodium acetate (NaC2H3O2), and weigh the. 4. Place a beaker on a magnetic stirrer and insert a probe of the previously calibrated pH meter into the beaker. 5. Add a little bit of the sodium acetate at a time, stirring gently with a magnetic bar to dissolve the salt until the pH is 5.0. In theory, this will take 0.216 g of sodium acetate. 6. Weigh the beaker with the remaining sodium acetate. 7. Quantitatively transfer the buffer solution to a 100 mL volumetric flask. 8. Add distilled water up to the mark. Cap and invert the flask to mix thoroughly. Data Analysis 1. Using the masses of sodium acetate and the beakers you weighed in, determine the exact mass of sodium acetate you added to the buffer. (2 marks) 2. Calculate the mass of sodium acetate (NaC2H3O2) that must be added to make 100 mL of an acetic acid/acetate buffer at pH = 5.0, given that you will use 5.0 mL of 0.50 M acetic acid. You must perform this calculation as part of your pre-lab work. (3 marks ) 3. According to the theoretical amount value calculated and actual of sodium acetate used, calculate percent error involved. Give a reason why this error occurred? (1.5 mark) TESTING THE BUFFERING CAPACITY Aim: To conduct a potentiometric titration on a prepared buffer solution and determine its buffering capacity on both acid and base. Introduction Even though a buffer will resist a change in pH, eventually when enough of either an acid or a base is added to, its buffering will be destroyed. The amount of acid or base needed to change the pH of a buffer is known as the "buffering capacity." You will measure the buffering capacity of the buffers you prepared by monitoring the titration with a pH meter. Part B: Buffering 1. Obtain 25 mL aliquot of the prepared buffer solution into 100 mL beaker. 3. Fill a 50 mL burette with your Standardized NaOH solution 4. Use the pH meter to monitor the titration of the buffer until the pH changes 1 unit. 5. Run two trails: a quick one and a careful one. For the quick one, add NaOH in 1 mL increament. For the careful one, use your best judgment in adding titrant. Data Analysis 1. Calculate the acid buffer capacity. Show your working. (2 marks) 2. Calculate the acid buffer capacity. Show your working. (2 marks) Buffer Capacity in Acid = π΅πππππ ππ πππππ ππ π¨πππ (π―+) (πͺπππππ ππ ππ―)(π½πππππ ππ π©πππππ ππ π³) Buffer Capacity in Base = π΅πππππ ππ πππππ ππ π¨πππ (πΆπ―−) (πͺπππππ ππ ππ―)(π½πππππ ππ π©πππππ ππ π³) MULUNGUSHI UNIVERSITY PURSUING THE FRONTIERS OF KNOWLEDGE PROGRAM: ……………………………………………………………………………………………………………….............. COURSE NAME ……………………………………………… COURSE CODE ……………………………………………….. STUDENT NAME …………………………………………………………………… STUDENT ID …………………………… NAME OF LAB INSTRUCTOR …………………………………………………………………………………................... EXPERIMENT NO. ………………………… DATE OF EXPERIMENT DUE DATE 27TH SEPTEMBER 2022 TITLE: ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… Aims: ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………….. Introduction: ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………. ……………………………………………………………………………………………………….......................................... .................................................................................................................................................. .................................................................................................................................................. Apparatus: ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… Procedure: ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………… Data Collection (1 mark) Mass of empty beaker Mass of Sodium Acetate Data Analysis I. Theoretical mass of sodium acetate required for buffer preparation. (3 mark) II. Percent error and give one reason for error occurred. (2.5 mark) Conclusion …………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………. Safety Attire compliancy (1.5 marks) οΆ Laboratory coat οΆ Laboratory Safety goggles οΆ Close toed shoe
