CHM 132 Lab Manual.pdf

Concentration will drop drastically and an

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concentration will drop drastically, and an appreciable voltage change will result. You will add ammonia to a solution containing Cu 2+ aq resulting in the formation of a complex as illustrated below. Cu 2 + ( aq ) + 4 NH 3 ( aq ) Cu NH 3 ( ) 4 + ( aq ) K f (4.18) Using Equation (4.17), you can calculate the concentration of Cu 2+ (aq) in the solution in the presence of the complex ion and use the results to calculate the formation constant, K f . Adding ammonia to a solution containing Ag + (aq) results in the formation of an Ag(NH 3 ) 2 + (aq) complex whose formation constant you also will determine. In an analogous experiment, the solubility product constant of AgCl will be determined. For this experiment the Ag electrode will be placed in a solution of known Cl - ion concentration that is in equilibrium with solid AgCl . From the measured cell potential and Equation (4.17), one can calculate the value of the Ag + (aq) concentration in the chloride-containing solution. Knowing the concentrations of Ag + (aq) and Cl (aq) in a solution in equilibrium with AgCl (s) allows one to find K sp for AgCl . Pre-Lab Questions: 1. What is the purpose of the agar salt bridge used in this experiment? 2. Write down the Nernst Equation and define the terms. 3. What effect does addition of ammonia ( NH 3 ) have on a solution of Cu 2+ (aq) ions? Experimental Procedures Part A The first step in this experiment involves setting up a series of electrochemical cells with “ salt bridge s” connecting the half-cells. These salt bridges will be made with agar, which consists of a chain of sugar molecules, and provides a gel with high water content. The water in the gel can contain high concentrations of salt (KNO 3 ), allowing ionic conduction through the gel without rapid diffusion of ions outside of the gel. Figure 3: Basic cell setup

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Chemistry 132 Lab Manual Page 44 The cell depicted in Fig. 3 is to be used in measuring the effect of complex ion formation on cell potential, to determine K sp for AgCl , and to perform a potentiometric titration. The agar / water / KNO 3 gel serves as a salt bridge. Agar is shown at the bottom of the crucible. When measuring cell potentials, there will be several crucibles in the crystallizing dish. Agar / KNO 3 Salt bridges a) Pour 50 ml of deionized water from a graduated cylinder into a 150-mL beaker and bring the water to a boil. b) Add 1 g agar to the water and stir the solution for 2-3 minutes. Reduce the heat so that the solution is hot, but not boiling strongly (avoid an agar “latte” effect with excessive frothing). c) Add 5 g of KNO 3 to this solution and stir until the salt has dissolved. You may use the glass rod for stirring (Use tongs to hold the beaker.) d) Remove the beaker from the hotplate and allow the solution to cool on the bench for two to three minutes. e) Place the crucibles on a piece of smooth aluminum foil in a line perpendicular to the edge of the benchtop (for easier pouring). The hot liquid will come through the bottom of the crucible, so please place the crucibles on aluminum foil, or in ice (until cool).
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