SECTION II, Part A部分
Answer Questions 1, 2, and 3. The Section II score weighting for each question is 20 percent.
1. Answer the following questions about the solubility of some fluoride salts of alkaline earth metals.
(a) A student prepares 100. mL of a saturated solution of by adding 0.50 g of solid to 100. mL of distilled water at 25°C and stirring until no more solid dissolves. (Assume that the volume of the undissolved is negligibly small.) The saturated solution is analyzed, and it is determined that in the solution is
(i) Write the chemical equation for the dissolving of solid in water.
(ii) Calculate the number of moles of that dissolved.
(iii) Determine the value of the solubility-product constant,
(b) A beaker contains 500. mL of a solution in which both and are present at a concentration of 0.10 M at 25°C. A student intends to separate the ions by adding 0.20 M NaF solution one drop at a time from a buret. At 25°C the value of the value of Ksp for
(i) Which salt will precipitate first,？ Justify your answer. For parts (b)(ii) and (b)(iii) below, assume that the addition of the NaF solution does not significantly affect
the total volume of the liquid in the beaker.
(ii) Calculate the minimum concentration of necessary to initiate precipitation of the salt selected in part (b)(i).
(iii) Calculate the minimum volume of 0.20 M NaF that must be added to the beaker to initiate precipitation of the salt selected in part (b)(i).
(c) There are several ways to dissolve salts that have limited solubility. Describe one procedure to redissolve the precipitate formed in part (b).
2. Answer the following questions involving the stoichiometry and thermodynamics of reactions containing aluminum species.
An electrolytic cell produces 235 g of Al(l) according to the equation above.
(a) Calculate the number of moles of electrons that must be transferred in the cell to produce the 235 g of Al(l).
(b) A steady current of 152 amp was used during the process. Determine the amount of time, in seconds, that was needed to produce the Al(l) .
(c) Calculate the volume of , measured at 301 K and 0.952 atm, that is produced in the process.
(d) For the electrolytic cell to operate, the must be in the liquid state rather than in the solid state. Explain.
When Al(s) is placed in a concentrated solution of KOH at 25°C, the reaction represented below occurs.
(e) Using the table of standard reduction potentials shown above, calculate the following.
(i) E°, in volts, for the formation of
(ii) DG°, in for the formation of
3. A student was assigned the task of determining the enthalpy change for the reaction between solid MgO and aqueous HCl represented by the net-ionic equation above. The student uses a polystyrene cup calorimeter and performs four trials. Data for each trial are shown in the table below.
(b) The data in one of the trials is inconsistent with the data in the other three trials. Identify the trial with inconsistent data and draw a line through the data from that trial in the table above. Explain how you identified the inconsistent data.
For parts (c) and (d), use the data from one of the other three trials (i.e., not from the trial you identified in part (b) above). Assume the calorimeter has a negligible heat capacity and that the specific heat of the contents of the calorimeter is 4.18 J/(g·C°). Assume that the density of the HCl(aq) is 1.0 g/mL.
(c) Calculate the magnitude of q, the thermal energy change, when the MgO was added to the 1.0 M HCl(aq).Include units with your answer.
(d) Determine the student’s experimental value of △H° for the reaction between MgO and HCl in units of。
(e) Enthalpies of formation for substances involved in the reaction are shown in the table below. Using the
information in the table, determine the accepted value of △H° for the reaction between MgO(s) and HCl(aq).
(f) The accepted value and the experimental value do not agree. If the calorimeter leaked heat energy to the environment, would it help account for the discrepancy between the values? Explain.
SECTION II, Part B部分