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1. Introduction: A buffer is a solution that resists changes in pH when a small amount of a strong acid or base is added to it. The goal of this lab is to prepare a buffer with a known pH and test its response to the addition of strong acid and base. The smaller the change in pH(ΔpH) when strong acid or base is added, the larger the buffer capacity. The first part of this lab is to calculate how to prepare the buffer. This is done using the buffer preparation equations shown in the sample calculations and is partly based on using the Henderson-Hasseloalch (H.H.) equation, pH=pKa+log[WA][CWB]​ or pH=pKa+log molWA molCWB​ or pH=pKa−log molWA mmolCWB​ where pH=−log[H3​O−],pK4​=−log(K2​),CWB is the conjugate weak base and WA is the weak acid. As shown this equation can be used in units of either concentrations (usually molarity) or moles of the conjugate weak base and weak acid. After being assigned a pH, a buffer "system" needs to be chosen. A buffer system consists of a weak acid and its conjugate base. The best choice for making the buffer is to choose a weak acid that has a pK, value as close as possible to the pH of the buffer being prepared. This will require determining the pK4​ values for all of the weak acids in the "Materials Available" part of the Experimental section of this lab. For example, the first material listed is ammonium chloride. NH.CL. The ammonium ion is a weak acid with a Ks ​ value of 5.6×10−10 and a conjugate weak base of NH3​. Ammonium's pKs ​ value is 9.25. If you've been assigned a pH value very close to 9.25, then a buffer system of NH4​∗NH3​ is a great choice because your assigned pH and the buffer's pK, value are so close. If not, then keep going The second material listed is sodium acetate with three waters of hydration, NaC2​H3​O2​⋅3H=O. The acetate ion is a weak base. Its conjugate weak acid is acetic acid, HC2​H3​O2​. Acetic acid's pK, value is 4.74. If you've been assigned a pH very close to 4.74, then a buffer system of HC2​H3​O2​/C2​H3​O2​ is a great buffer system. When working with buffers, it's okay to start with either the weak acid or the weak base. Continue calculating the values of pK2​ for the weak acid in each buffer system that can be created from the Materials Available section until you find the pKN​ value that is closest to the pH of the buffer you've. been assigned to make. Remember that it could be the conjugate base that is listed in the Materials Available section, in which case you'll have to determine what the weak acid is, and then determine its K. and pKs​ values. Once the buffer system has been identified, it's worth taking a minute to discuss how we're going to prepare our buffer. The simplest way is to take a certain number of grams (and therefore moles) of the weak acid and a certain number of grams of the weak base, mix them together and dilute to 100.0 mL. Our approach will be slightly different and slightly more complicated, but still an excellent approach. We will start by getting grams of either the weak acid or the weak base. Let's say we get grams of NHCL, which contains a weak acid NH4∗​. Then we will add NaOH, a strong base. The NaOH reacts with the NH4​∗ to completion (no NaOH left over) to make NH3​ while leaving some NH2​+left over (it's the "exces reactant"). Then, with NH3​ and NH4​+present, you've made a buffer! Awesome!
pK, ₹ assigned pH within ±1pH unit The assigned pH is 4.32. Assume the Materials Available in the experimental section only included sodium acetate and sodium oxalate. If you determine their K, values, you find (among other things) that sodium oxalate (Na2​C2​O4​) has as its anion the weak base C2​O42​. Its conjugate weak acid HC2​O4−​has a pK4​ value of 4.21, very close to my assigned pH I On the other hand, NaC2​H3​O3​ is a conjugate weak base to HC2​H3​O2​ (acetic acid), which has a pK, of 4.74, not as close. Our approach to preparing a buffer will be to start with grams of Nay C2​O4​, which provide C2​O2, our weak base. Then, we'll add a little HCl to the C2​O22​. These two will react to completion (no HCl left over) to make HC2​O2​ " while leaving some C2​O42−​ left over (C2​O42−​ is the "excess reactant" ). Then, with both HC2​O2−​and C2​O42​ presen, we'll have prepared a buffer! So we'll start with solid Na2​C2​O4​ (our conjugate weak base, CWB ) and add HCl to generate our HC2​O4−​(weak acid, WA) via the following reaction: Na2​C2​O4​+HCl→NaHC2​O4​+NaCl or assuming that we can ignore Na+as a spectator ion and treat HCl as the strong acid it is (H3​O+): CO42​(aq)+H3​O+(aq)→HC2​O4−​(aq)+H2​O(1) Our task in the next steps is to calculate the mass of solid Na2​C2​O4​ to be taken, and the amount of 1.00MHCl to be added to this salt to make a buffer at pH=4.32. Depending upon your assigned buffer system, you may instead be using solid WA and adding NaOH to generate the CWB, but the calculations are very similar. For us:  WA =HC2​O4−​ CWB =C2​O42−​ solid salt form of CWB=Na2​C2​O4​ 2. Use H. −H. to calculate the conjugate base / conjugate acid ratio for the given pH. We will us the version of the H,−H. equation in moles pH=pK0​+logmmolWAmmolCWB​→B=mmol CWB:A= mmol WA pH=pKa​+logAB​→pKa​=4.21 : assigned pH=4.32pH−pKα​=logAB​→4.32−4.21=0.11=logAB​AB​=mmolWAmmolCWB​=100.11=1.29 Buffer Prep Equation #1​
3. Based on the assigned concentration and volume of buffer, calculate total mol of weak acid + mol of weak base in the buffer. Our assigned values for our buffer are 100 mL and 0.100M. 0.100 L×0.100Lmol​=0.0100 mol×1000mmol/mol=10.0mmol Doing a limited reactant problem, knowing that the acid will be the limited reactant, 4. Check arithmetic 5. Determine the grams of CWB needed and the mL of HCl needed to prepare this buffer. We can use solid Na2​C2​O4​ that supplies our CWB(C2​O42​). We will start with 0.010 mol of solid Na2​C2​O4​ (for which it's easy to calculate the number of grams of Na2​C2​O4​ needed, right?) and react it with the strong acid to create the weak acid, HC2​O7​. Starting initially with 10mmol of C2​O4​2− (from Na2​C2​O4​ ), we must end with mmol 5.63C2​O42​ and 4.37mmolHC2​O4−​. Our ICE table looks like this so far: Filling in the ICE table allows us to determine the initial number of mol of H3​O−, and therefore the number of moles of HCl, to add to create our buffer. Filling in the empty spaces: Calculate the grams of solid Na2​C2​O4​ from the initial mol of Na2​C2​O4​(0.010 mol) and the mL of HCl from the initial mol of H3​O+and the concentration of HCl,1.00M.
6. Make the buffer. Add solid Na2​CsO4​ and about 30 mL water to dissolve the sall fully, and then add the mL. of HCl. Make volume up 10100 mL in a volumetrie flask. STOP HERE during your pre-lab calculationst! Summarize the results in Table 2 for your instructor to review. The rest of the calculations can be done later. B. Calculation of the theoretically expected pH change for the full-strength buffer. Ex. How will the pH of the above buffer change upon addition of 0.200 mL of 1MHCl to 20.0 mL of the full-strength buffer? 1. Calculate the mol of WA and CWB in 20.0 mL of the total 100 mL. As prepared the buffer has 4.37mmol of HCM2−​(WA) and 5.63mmol of C2​O42​ - (CWB) in the total ) 100.0 mL of buffer. We will be testing 20.0 mL of buffer with: Similarly, there are 1.126mmolC2​O42−​ in 20.0 mL of buffer- 2. Use the mol of WA and CWB as the initial values in a mol ICE table, along with mol HCL. 3. Solve for pH using Henderson Hasselbalch. pH=pKa​+logmmolWAmmolCWB​pH=4.29+log1.074 mmol HC2​O4−​0.926mmolCO42−​​=4.23 4. Calculate the change in pH as ΔpH=pHinal ​−pH intial. The initial pH is the pH of the full-strength buffer before any acid is added, 4.32. The final pH is the pH after the acid is added, also 4.23. Therefore, in this case, ΔpH=−0.09. C. Calculation of the mol of WA in the "Half-strength" buffer. To prepare 20.0 mL of the half-strength buffer, 10.0 mL of the full-strength buffer (with the same number of moles of WA and CWB as calculated in B above), and 10.0 mL of water are mixed. This means we have half of the moles that were in 20.0 mL of the full-strength buffer.
III. Experimental: A. Equipment and Materinls needed: From locker: 100 mL wol. flask, 10 mL graduated pipet, 10 mL and 100 mL gruduated cylinder. From the stock room: pH meter and electrode (after your buffer calculation has been checked by your instructor), rubber bulb for pipet, micro pipette. Reagents Available: Formula weights of the salts are given here. You should make sure the formulae and the numbers given here agree with the ones on the bottle. Depending on the number of waters of hydration (for e.g., CuSO4​⋅5H2​O has 5 waters of hydration), the molar mass of the hydrated salt will vary. Strong Acid: 1.00MHCl Strong Base: 1.00MNaOH B. Disposal All waste must be collected and tested for the proper pH range (4-10) prior to disposal. C. Before Starting Experimental Work (Before Class) 1. In your notebook, enter the experiment titie, date, your name and name of partner. 2. Write the purposes of the lab. 3. Write an executive summary of the procedures described in this lab. 4. For the substances listed above, complete Table 1. This requires you to: - identify the anion or cation that has hydrolysis properties - determine if it can act as an acid, base, or both - identify the Ka and pKa of the ion itself if it is acidic or its conjugate acid if it is basic: - Highlight or circle the substance that would be best for preparation of your assigned buffer. Place the information in Table 1 5. Have the Buffer Design Calculations completed for your buffer. There is an extensive sample calculation for you to follow so that you can do this. See below. If you have not been given a pH for the calculations, ask your instructor. If all all else fails, work through the calculations for pH=5.00. Assume that the volume is 100.0 mL and the total concentration of acid and base is 0.100M. Fill in Table 2.
D. Procedere 1. Buffer Preparation. 1. If you have not done so already, convleie the pre-lab caleulations by solving the Buffer Prep equations for this lab (example shown in Sample Calculations section) for your assigned plf. (If you did not get a pH assigned to you, ask your instructor or use 5.00 as your assizned pH as a last resort.) At the completion of the calculations, you should know how many grams of either weak acid or weak base you need to weigh and how many mL of either NaOH or HCl you need to add to create your buffer, Summarize your calculations by completing Table 1 in your notebook. You will not be allowed to begin lab antil your Table 1 \& 2 values and associated calculations have been verified by your instructor? 2. Weigh out the solid to be used in your buffer. Add the solid to a 100 mL volurnetric flask. Fill the flask approximately half foll with D1 water and mix until the solid is dissolved. 3. After the solid is fully dissolved, add the HCl or NaOH to the volumetric flask. Use the graduated pinette to measure the HCl or NaOH. 4. Fill the volumetric flask with DI water so that the bottom of the meniscus is in line with the white line on the neck of the volumetric flask. In order to add the fast few drops, you can use one of the droppers in your locker. S. Place the top, a slopper, or Parafilm over the top of the volumetric flask. Invert until completely mixed {=20 times ). 6. Calibrate the pH meter with pH=4,7 and 10 buffers, as described in the pH experiment. 7. Pour all of your buffer into a 200−250 mL beaker and measure its pH. If the pH of your buffer is within 0.50pH units of the correct value. continue with Procedure 2 . If your buffer is not within 0.50pH unit of your assigned pH, speak with your instructor. E. Procedure 2: Measuring Buffer Capacity for the Full Strength Buffer. There are many definitions of buffer capacity; we will use the definition as the change in pH(ΔpH) 1.0 mL of 1.0M acid or 1.0 mL of 1.0M base is added to 100 mL of buffer (equivalently. 0.200 mL of 1.0M acid or base added to 20 mL of buffer). Hence, ΔpH=pH firi-i −pH inina The undiluted buffer will be the most full-strength buffer. Therefore, we will refer to it as the "full strength" buffer. 1. Create Table 2 in your notebook to record the data that you will be collecting. 2. Pipette out two 20.0 mL aliquots of the full strength buffer as accurately as possible into two 50n beakers. Do not use larger beakers or the solution depth may not cover the pH electrode. 3. To one aliquot, add 0.200 mL of 1.00MHCl with a micropipette. Stir the solution and record the new pH. 4. Rinse the pH electrode and place into the second aliquot. Add 0.200 mL of 1.00MNaOH. Stir a record the new pH. 5. State the buffer capacity for acid and for base (i.e., ΔpH for acid addition and ΔpH for base addi in your lab notebook. For the addition of acid the ApH will be negative. F. Procedure 3: Measuring the Effect of Dilution on the pH and the Buffer Capacity of the Buf The Half-strength (Half-Strength) Buffer 1. Re-measure the pH of your full strength buffer, and record it as the pH of the full-strength buff before dilution. Has the pH of the full-strength buffer changed with time? Calculate how much any it has changed.
2. Pipette a 10.0 mL aliquot of your full-strength buffer into each of two fresh, clean 50 mL beakers using partner "A's" pipette. (Keep this pipette for use with the full-strength buffer in the following procedure) Using the pipette from partner "B", add 10.0 mL delonized water to each, stirring well with a stirring rod. These beakers now contain a "half-strength buffer". Measure the pH of the halfstrength buffer in one of the two beakers. Record this as the pH of the half-strength buffer after dilution. Is the pH of the diluted buffer different from the pH of the original full-strength buffer measured above? Calculate how much if any it has changed. 3. To one of the two beakers containing the half-strength buffer, add 0.200 mL of 1.0MHCl. Stir the solution and record the new pH as the pH after acid addition for the half-strength buffer. 4. Rinse the pH electrode and place into the other beaker containing the half-strength buffer. Add 0.200 mL of 1.0MNaOH. Stir the solution and record the new pH as the pH after base addition for the half-strength buffer. 5. State the buffer capacity (Apif) of the diluted buffer upon addition of acid or base in your notebook by comparing the pH of the diluted buffer before and after the addition of the acid or base. The Quarter-strength Buffer 1. Re-measure the pH of your full-strength buffer, and record it as the pH of the quarter-strength buffer before dilution. Has the pH of the full-strength buffer changed with time? Calculate how much if any it has changed. 2. Pipette a 5.0 mL portion of your full-strength buffer (with the pipette from partner " A ") into each of two fresh, clean 50 mL beakers. Using a pipette (from partner "B"). add 15 mL deionized water to each, stirring well with a stirring rod. These beakers now contain the "quarter-strength buffer". Measure the pH of the quarter-strength buffer in one of the two beakers. Record this as pH of the quarter-strength buffer after dilution. Is the pH of the diluted buffer different from the pH of the original full-strength buffer measured above? Calculate how much if any it has changed. 3. Repeat steps 3−5 with the two beakers of quarter-strength buffer to determine its buffer capacity to acid and base. State the buffer capacity for acid and for base. G Procedure 4. Neutralization of Chemicals Used in This Experiment 1. As assigned by the instructor, neutralize the chemicals used in this experiment. When the chemicals are within the range of pH=4−10 (as tested using your pH meter), then pour the chemicals down the drain and rinse the sink afterwards for 2-3 minutes with tap water. IV. Further Instructions A. Required Calculations 1. For the "full-strength" buffer: Calculate the expected change in pH [the expected buffer capacity (ΔpH)] for the addition of 0.200 mL of 1.00MHCl to the full-strength buffer. 2. For the "full-strength" buffer: Calculate the expected change in pH [the expected buffer capacity (ΔpH)] for the addition of 0.200 mL of 1.00MNaOH to the full-strength buffer. 3. For the "half-strength" buffer: Calculate the expected change in pH [the expected buffer capacity (ΔpH)] for the addition of 0.200 mL of 1.00MHCl to the half-strength buffer. 4. For the "half-strength buffer": Calculate the expected change in pH [the expected buffer capacity (ΔpH)] for the addition of 0.200 mL of 1.00MNaOH to the half-strength buffer. 5. For the "quarter-strength" buffer: Calculate the expected change in pH [the expected buffer capacity (ΔpH)] for the addition of 0.200 mL of 1.00MHCl to the quarter-strength buffer. 6. For the "quarter-strength buffer": Calculate the expected change in pH the expected buffer