exp7 - Anusha Vadlamudi Partner: Christen Burns TA: Lindsay...

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Anusha Vadlamudi Partner: Christen Burns TA: Lindsay Aldridge Chem 241 Lab; Section 401; Room 300 Experiment 7: Preparation and Investigation of Buffers (5/15/08)
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Honor Code Pledge: I pledge that no unauthorized assistance has been given or received in the completion of the work presented in this report. Results: Experiment 1: a. Method 1: Combining an acid and its conjugate base Volume of liquid acetic acid used = 1.14 mL Mass of Sodium acetate used = 4.773 g Final pH = 5.83 b. Method 2: Partial Neutralization of a weak acid with a strong base Volume of liquid acetic acid used = 1.14 mL Volume of NaOH used = 12.74 mL Final pH = 5.09 c. Method 3: Real world buffer preparation Initial pH = 3.41 pH after 27 drops = 4.95; 28 drops = 5.01 Final pH after transfer and dilution = 4.39 Percent error calculation for method 1: % Error = 100 value) (true value) true - value (exp x = ((5.83-5.00)/5.00) x 100 = 16.6% Method used Target pH Actual pH % error 1 5.00 5.83 16.6 2 5.00 5.09 1.8 3 5.00 4.39 12.2 Table 1: Comparison of actual pH with target pH using various methods Experiment 2: Assigned pH = 9.64
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Of the available reagents, Ammonium Ion with a pKa of 9.245 is the closest pKa to the given pH. Since it’s a weak acid, we can use the partial neutralization method by adding strong base. The strong base we chose among the available reagents was NaOH. Attempt #1 : O H NH NaOH Cl NH 2 3 4 + + Using the Henderson-Hasselbalch equation, the amount of each reagent needed can be calculated. [ ] [ ] ) 005 (. ) ( 245 . 9 64 . 9 4 3 x x Log Cl NH NH Log pK pH a - + = + = 3 565 . 3 3 - = = E x NH moles [ ] 3 436 . 1 ) 005 (. 4 - = - = E x Cl NH moles Since moles 3 NH produced equals moles of NaOH added, we need to add 3.565E-3 moles of NaOH. Since the available reagent is 1.0 M NaOH, this can be converted to mL mL mol mL molx E 565 . 3 1 1000 3 565 . 3 = - Since the ammonium chloride available is in solid form, we need to convert 1.436E-3 moles to grams. = - mol g molx E 49 . 53 3 436 . 1 7.681E-2 g Method: Partial Neutralization of a weak acid with a strong base Volume of NaOH used = 3.56 mL Mass of Ammonium Chloride used = .077g
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Final pH = 11.6 Experiment 3 # of drops added of 1.0 M HCl or 1.0 M NaOH Buffer # 0 1 2 3 4 5 1 3.39 3.24 3.19 3.19 2.99 2.98 2 3.6 3.35 3.1 2.8 2.5 2.4 3 4.02 4.02 3.81 3.65 3.5 3.19 4 Initial: 4.44 1 d HCl 4.39 2 d HCl 4.34 Initial 4.44 1 d NaOH 4.49 2 d NaOH 4.54 5 5.01 5.11 5.16 5.32 5.37 5.47 6 5.59 5.69 5.79 5.99 6.19 6.79 7 5.88 6.23 10.57 10.97 11.17 11.26 Table 2: pH of a buffer solution after addition of HCl or NaOH for each buffer # All the buffer solutions were graphed to find the linear regression in order to find the
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exp7 - Anusha Vadlamudi Partner: Christen Burns TA: Lindsay...

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