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AP Chemistry 1981 Free Response Questions

AP Chemistry 1981 Free Response Questions - 1981 Free...

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1981 Free Response Questions 1) Ammonia hydrogen sulfide is a crystalline solid that decomposes as follows: NH 4 HS(s) -----> NH 3 (g) + H 2 S(g) (a) Some solid NH 4 HS is placed in an evacuated vessel at 25 °C. After equilibrium is attained , the total pressure inside the vessel is found to be 0.659 atmosphere. Some solid NH 4 HS remains in the vessel at equilibrium. For this decomposition, write the expression for K p and calculate its numerical value at 25 °C. (b) Some extra NH 3 gas is injected into the vessel containing the sample described in part (a). When equilibrium is reestablished at 25 °C, the partial pressure of NH 3 is twice the partial pressure of H 2 S. Calculate the numerical value of the partial pressure of NH 3 and the partial pressure of H 2 S in the vessel after the NH 3 has been added and equilibrium has been reestablished. (c) In a different experiment, NH 3 gas and H 2 S gas are introduced into an empty 1.00-liter vessel at 25 °C. The initial partial pressure of each gas is 0.500 atmosphere. Calculate the number of moles of solid NH 4 HS that is present when equilibrium is established. 2) A(aq) + 2 B(aq) ----> 3 C(aq) + D(aq)
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For the reaction above, carried out in solution at 30 °C , the following kinetic data were obtained: Experiment Initial Ra Reacti mole liter¯ A o B o 1 0.240 0.480 8.00 2 0.240 0.120 2.00 3 0.360 0.240 9.00 4 0.120 0.120 0.500 5 0.240 0.0600 1.00 6 0.0.140 1.35 ? (a) Write the rate-law expression for this reaction. (b) Calculate the value of the specific rate constant k at 30 °C and specify its units. (c) Calculate the value of the initial rate of this reaction at 30 °C for the initial concentrations shown in experiment 6. (d) Assume that the reaction goes to completion. Under the conditions specified for experiment 2, what would be the final molar concentration of C? 3) A 1.2156-gram sample of a mixture of CaCO 3 and Na 2 SO 4 was analyzed by dissolving the sample and completely precipitating the Ca 2+ as CaC 2 O 4 . The CaC 2 O 4 was dissolved in sulfuric acid and the resulting H 2 C 2 O 4 was titrated with a standard KMnO 4 solution. (a) On a page of your answer booklet, write the balanced equation for the titration reaction, shown balanced below.
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MnO 4 ¯ + H 2 C 2 O 4 + H + ----> Mn 2+ + CO 2 + H 2 O Indicate which substance is the oxidizing agent and which substance is the reducing agent. (b) The titration of the H 2 C 2 O 4 obtained required 35.62 milliliters of 0.1092-molar MnO 4 ¯ solution. Calculate the number of moles of H 2 C 2 O 4 that reacted with the MnO 4 ¯. (c) Calculate the number of moles of CaCO 3 in the original sample. (d) Calculate the percentage by weight of CaCO 3 in the original sample. 4) Use appropriate ionic and molecular formulas to show the reactants and the products for the following, each of which occurs in aqueous solution except as indicated. Omit formulas for any ionic or molecular species that do not take part in the reaction. You need not balance. In all cases a reaction occurs.
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