Using Buffers

CHM 101A Using Buffers Final Report Student Name Student ID Lesson Using Buffers Institution Session Course Instructor Test Your Knowledge 1. Categorize each statement as true or false. True False 1 2 Chemical buffers have specific ranges and capacities. Buffers are effective at resisting pH changes when large amounts of acid or base are added to a solution. Chemical buffers are important to industrial production and to living systems. The buffer capacity is the pH range that is maintained when acids and bases are added to a solution. 2. Match each term with the best description. Solution that resists pH changes Compound that fully ionizes in water Product of the ionization of an acid in water Compound that only partially ionizes in water 1 Chemical buffer 2 Strong acid 3 Conjugate base 4 Weak acid 3. Identify the variables of the equation . Acid ionization constant 1 Ka Weak acid concentration 2 [HA] Acidity of solution 3 pH Conjugate base concentration 4 [A -] Exploration 1. A chemical buffer is a solution that resists _____ when small quantities of an acid or base are added to it. pH changes pressure changes thermal changes 2. The function of a buffer is to keep a solution neutral when small amounts of acids or bases are added. True False 3. The _____ is the amount of acid or base that can be added to a solution before the pH significantly changes. buffer capacity buffer range buffer zone 4. Buffers typically consist of a _____ and _____. strong acid; its conjugate base weak acid; its conjugate base strong acid; a strong base 5. The base component of the acetic acid buffer system is _____. acetate ammonia sodium hydroxide 6. The concentration of an acid in the Henderson-Hasselbalch equation is represented by _____. pKa [A -] [HA] 7. The Henderson-Hasselbalch equation is used to _____. calculate the pH of a buffered solution determine the acid and base concentrations required to prepare a buffer calculate the effect of adding additional acid or base to a buffer All of the above Exercise 1 1. Describe the purpose of a buffer. A buffer is a mixture of molecules that or bind order of to maintain relatively stableorpH. function of a buffer is not to keep a solution neutral. Its function is release to minimize the in amount change ina pH when base acidThe is added to solution. 2. Write the chemical equations for the neutralization reactions that occurred when HCl and NaOH were added to the buffer solution. Buffer Solution: CH3COOH(aq)+NaOH(aq)CH3CooNa(aq)+H2O(l) The buffer solution contains aceticacetate ac id and its weak odium acetate as its conjugate base. If we add strong HCI to the buffer solution above, the sodium would reactacid withand HCIsas in the chemical equation below: HCI(aq)+CH3COONa(s)CH3COO H(aq)+NaCl(s) If we add sodium hydroxide to the buffer solution the acetic ac id in the buffer would react as in the following chemical e=reaction equation: NaOH(aq)+CH3COOH(aq)CH3COONa(s)+H2O(l) 3. How do the results in Data Tables 1 and 2 support the role of a buffer? In the presence of a buffer, the addition of 0.1M HCl or 0.1 M NaOH resulted in no change in pH. The solution was capable of maintaining a nearly constant pH when the smaller/ dilute concentrations of acid or base were added. 4. Describe the buffer capacity of the acetic acid buffer solution in relation to the addition of both concentrated and dilute acids and bases. Reference the results in Data Tables 1,2,3, and 4 in your answer. In the presence of the acetic acid buffer solution, addition of dilute acid and dilute base did not affect the pH of the solution, as shown in tables 1 and 2. However, addition of a higher concentration of the strong acid and base altered the pH level significantly. The concentrated solutions of either the acid or the base has a significant effect in the pH of acetic acid buffer solution while the diluted acids and bases had no effect on the pH showing the buffering capacity of the acetic acid buffer solution could withstand only small amounts of the dilute acids and bases. 5. Did distilled water act as a buffer in the experiment? Use the results in Data Tables 5 and 6 to support your answer. Distilled water did not act as a buffer due to the fact it was not able to withstand or buffer the solution upon addition of an acid or base. 6. An acetic acid buffer solution is required to have a pH of 5.27. You have a solution that contains 0.01 mol of acetic acid. What molarity of sodium acetate will you need to add to the solution? The pKa of acetic acid is 4.74. Show all calculations in your answer. pH = pKa + log [A-]/[HA] PH = PKa + log[CH3COONa]/1 log[CH3COONa] = 5.27-4.74 log[CH3COONa] = 0.53 No of moles of CH3COONa = 100.53 = 3.3884 moles Data Table 1: Adding 0.1 M HCl from D1 to A1 Number of Drops pH of Solution 0 6.0 2 6.0 4 6.0 6 6.0 8 6.0 10 6.0 12 6.0 14 6.0 16 6.0 Data Table 2: Adding 0.1 M NaOH from D6 to A6 Number of Drops pH of Solution 0 6.0 2 6.0 4 6.0 6 6.0 8 6.0 10 6.0 12 6.0 14 6.0 16 6.0 Data Table 3: Adding 6 M HCl from Pipet into B1 Number of Drops pH of Solution 0 6.0 2 6.0 4 4.0 6 2.0 8 2.0 10 2.0 Data Table 4: Adding 6 M NaOH from Pipet into B6 Number of Drops pH of Solution 0 6.0 2 6.0 4 6.0 6 8.0 8 10.0 10 12.0 Data Table 5: Adding 0.1 M HCl from D1 into C1 Number of Drops pH of Solution 0 6.0 2 4.0 4 4.0 6 4.0 8 4.0 10 4.0 Data Table 6: Adding 0.1 M NaOH from D6 into C6 Number of Drops pH of Solution 0 6.0 2 12.0 4 12.0 6 12.0 8 12.0 10 12.0 Competency Review 1. A chemical buffer is a solution that increases pH changes when small quantities of an acid or base are added. True False 2. Every buffer that is made has a certain _____ and _____. buffer temperature; buffer pressure buffer range; buffer capacity buffer zone; buffer action 3. Weak acids make better buffers than strong acids because they have _____. conjugate bases of reasonable strength weak conjugate bases low pH values 4. _____ is a weak acid present in commercial vinegar that can be used to create a buffer solution. Phosphoric acid Acetic acid Hydrochloric acid Formic acid 5. The pH of a buffered solution is determined by _____. the concentration of the weak acid the concentration of the conjugate base the acid ionization constant All of the above 6. pH = pKa if acid concentration is _____. higher than conjugate base concentration lower than conjugate base concentration equal to conjugate base concentration 7. _____ can be added to commercial vinegar to create an acetic acid buffer solution. Distilled water Hydrochloric acid Sodium hydroxide 8. Large volumes of concentrated acids and bases should be added to buffered solutions when testing buffer ranges and capacities. True False 9. Why would buffered solution tests be compared to distilled water? Distilled water is not a buffer and acts as a control. Distilled water is an effective buffer against strong acids. Distilled water is an effective buffer against strong bases. 10. Acetic acid buffers are most effective in resisting pH changes to _____. concentrated acids and bases diluted acids and bases Extension Questions 1. John needs to create a buffered solution at a pH of 3.5 for his biomedical laboratory. a. Using the chart below, determine the appropriate acid for this application. b. Research and state the conjugate base of the the acid. c. Describe how John would determine the amounts of acid and base needed to create the buffered solution. Buffer a. A buffer is prepared from the weak acid which has its pKa closest to the required pH of the resultant buffer. therefore, the weak acid to be used here would be lactic acid. b. Conjugate base of the acid would be sodium lactate. c. Using Hendersen-Hasselbalck equation, pH = pKa + log(base/acid) 3.5 = 3.86 + log(sodium lactate/lactic acid) (sodium lactate/lactic acid) = 0.44