Lab #1 Instructions

Lab #1 Instructions - EXPERIMENT I Acid Base Equilibrium...

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EXPERIMENT I Acid – Base Equilibrium and Buffers PURPOSE: Determine the pH of aqueous solutions of weak acids and bases using pH indicators, standardized buffer solutions, and a pH meter. Prepare a buffer of specified pH using a weak acid and the salt of its conjugate base. INTRODUCTION: This experiment is related to material covered in lecture last term and to topics being discussed currently. Its goal is to provide practical experience and to provide more meaning to the calculations which you are doing in lecture. To accomplish this, you will: calculate the theoretical pH of a variety of aqueous solutions containing weak acids and weak bases; use acid-base indicators to deduce a pH range for each of these solutions; determine the pH of an unknown using an acid-base indicator with buffer solutions of known pH; learn how to calibrate a pH meter; use the pH meter to measure the pH of solutions; prepare a buffer with a pH specified by the instructor, using available solutions in the laboratory. Part 1: pH Calculations You will be working with 9 solutions, your unknown, and an aqueous sample of your choosing. Before coming to lab, you should calculate the theoretical pH for each of the 9 known solutions as shown in Table 3, using the equilibrium constants given in Table 1, below. Complete calculations should be included in your report for each of the nine solutions. This portion of the exercise should be completed outside of laboratory and it is certainly appropriate to ask for assistance from your lecture or lab instructor. Although we will be working similar problems in lecture, you may wish to consult the text sections related to "acid-base equilibria" or "weak acids and bases" to complete this part. The equilibria, equilbrium constants, and pK's provided in Table 1 should be used in these calculations. ALL solutions are 0.100 M. Use the approximation that 0.100 – x 0.100 to start these calculations. If x turns out to be << 0.100, this is a good assumption and you do not have to use the quadratic formula in your calculations. However, if x > 0.005, it would be a good idea to use the quadratic formula to avoid appreciable errors. Indicate in your report which calculations required the use of the quadratic formula.
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_________________________________________________________________________ Table 1 Selected Equilibria Acetic Acid HC 2 H 3 O 2 H + + C 2 H 3 O 2 - K a = 1.76x10 -5 pK a = 4.75 Phosphoric Acid H 3 PO 4 H + + H 2 PO 4 - K a1 = 7.52x10 -3 pK a1 = 2.12 H 2 PO 4 - H + + HPO 4 2- K a2 = 6.23x10 -8 pK a2 = 7.21 HPO 4 2- H + + PO 4 3- K a3 = 2.20x10 -13 pK a3 = 12.67 Ammonia NH 3 + H 2 O NH 4 + + OH - K b = 1.79x10 -5 pK b = 4.75 ________________________________________________________________________ There are essentially 5 situations that you will need to consider. [1] Weak acid problem HA H + + A - K = [H ][A ] [HA] = (x)(x) (0.100- x) a +- Examples: 0.100 M HC 2 H 3 O 2 and 0.100 M H 3 PO 4 [2] Weak base problem B + H 2 O BH + + OH - [BH ][OH ] [B] (x)(x) (0.100- x) b Example: 0.100 M NH 3 [ 3] Salt of a weak acid A - + H 2 O HA + OH - K K [HA][OH ] [A ] (x)(x) (0.100- x) w a - - Example: 0.100 M NaC 2 H 3 O 2 and 0.100 M Na 3 PO 4 OR Salt of a weak base BH + + H 2 O B + H 3 O + K K [B][H O ] [BH ] (x)(x) (0.100- x) w b 3 + + Example: 0.100 M NH 4 Cl
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[ 4] Intermediate salts Examples: 0.100 M NaHPO 4 and 0.100 M Na 2 HPO 4 Notice in Table 1 that all of these species are involved in two equilibria. For example,
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Lab #1 Instructions - EXPERIMENT I Acid Base Equilibrium...

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