You will experimentally determine the K^c and K^t for dissolving insoluble Pbl_2

Question

You will experimentally determine the K^c and K^t for dissolving insoluble Pbl_2 in water. Determination of the K^c requires the equilibrium molarities of Pb^2+ and I^- in a saturated solution of Pbl_2. We need a solution in which a dynamic solubility equilibrium has been established, so a saturated solution of Pbl_2 in contact with excess solid Pbl_2 will be used. Using the stoichiometry of the dissolution process allows us to determine only one of the two. [Pb^2+] or [l^-] Pbl_2 Pb^2+ + 2i^- You will oxidize the iodide ion in the saturated Pbl_2 solution to elemental iodine, I_2, using nitrite ion in a slightly acidic medium. The absorbance of the iodine in the solution will be measured using a spectrophotometer. A second solution containing a known concentration of T will also be treated with acidified KNO_2. The absorbing species is the same for the saturated Pbl_2 solution and the known Kl solution Using Beer s Law. A = epsilon bc, the known concentration of l_2 from the Kl (aq), and the absorbance of the l_2 from the known solution, we can calculate the molar absorptivity, epsilon, for l_2. The calculated molar absorptivity and the absorbance of l_2 from the unknown solution can be used to calculate the equilibrium concentration of l_2, [l_2]+eq, in the diluted unknown solution. Do not add the potassium nitrite solution until you have access to a spectrophotometer. Once the l_2 is formed, it escapes from the solution over time. So absorbance measurements should be taken soon after the reaction is complete. The [l_2] we calculated is the concentration of the diluted solution. We added KNO_2 solution and HCI solution to convert the l^- into l_2. We must determine the concentration of I^- in the original Pbl_2 solution. It is necessary to work backwards from the measured [I_2] in the diluted solution to the [I^-] in the original saturated Pbl_2 solution. Using the original concentrations! iodide ion. [I^-], before the dilution, we can calculate the concentration of load(ll) ion. [Pb^2+].K_sp^c can be calculated using [Pb^2+] and [I^-]. The ionic strength of the solution can be calculated since we know the major concentrations of ions in the solution The activity coefficient. and the activity of each ion, a_x, can be calculated providing the necessary components to calculate K_sp^t. The thermodynamic equilbrium constant can be compared to an accepted value from Lange s Handbook. Write the half reaction for the oxidation of iodide ion. A student started with 3.00 mL of 4 56 Times 10^-3 M Kl solution. To this solution, she added 3.00 mL of 0.02 M KNO_2 and 0.900 mL of 1 M HCI. Calculate the initial concentration of I^- in the diluted solution. Write the chemical equation for the dissolving of lead(ll) iodide Give the equilibrium expression for the solubility product of lead(ll) iodide. If the equilibrium concentration of iodide ion. [I^-]_eq, in Pbl_2 solution equals 2.40 Times 10^-3 M, what is the equilibrium concentration for lead(ll) ion?

Explanation / Answer

1. Half reaction,

2I- <===> I2 + 2e-

2. moles of I- = 4.56 x 10^-3 M x 3 ml = 0.01368 mmol

concentration of [I-] in diluted solution = 0.01368 mmol/6.9 ml = 0.002 M

3. Equation for dissolution of PbI2,

PbI2 <==> Pb2+ + 2I-

4. Ksp for PbI2 = [Pb2+][I-]^2

5. Ksp for PbI2 = 6.5 x 10^-9

[Pb2+]eq = Ksp/[I-]^2

                = 6.5 x 10^-9/(2.40 x 10^-3)^2

                = 1.13 x 10^-3 M

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