Section II Part A
Answer Questions 1, 2, and 3. The Section II score weighting for each question is 20 percent.
1. A 1.22 g sample of a pure monoprotic acid, HA, was dissolved in distilled water. The HA solution was then titrated with 0.250 M NaOH. The pH was measured throughout the titration, and the equivalence point was reached when 40.0 mL of the NaOH solution had been added. The data from the titration are recorded in the table below.
(a) Explain how the data in the table above provide evidence that HA is a weak acid rather than a strong acid.
(b) Write the balanced net-ionic equation for the reaction that occurs when the solution of NaOH is added to the solution of HA .
(c) Calculate the number of moles of HA that were titrated.
(d) Calculate the molar mass of HA .
(e) Assume that the initial concentration of the HA solution (before any NaOH
solution was added) is 0.200 M.Determine the pH of the initial HA
(f) Calculate the value of  in the solution after 30.0 mL of NaOH solution is added and the total volume of the solution is 80.0 mL.
2. A sample of a pure, gaseous hydrocarbon is introduced into a previously evacuated rigid 1.00 L vessel. The pressure of the gas is 0.200 atm at a temperature of 127°C.
(a) Calculate the number of moles of the hydrocarbon in the vessel.
(b) is introduced into the same vessel containing the hydrocarbon. After the addition of the , the total pressure of the gas mixture in the vessel is 1.40 atm at 127°C. Calculate the partial pressure of in the vessel.
The mixture of the hydrocarbon and oxygen is sparked so that a complete combustion reaction occurs, producing and . The partial pressures of these gases at 127°C are 0.600 atm for and 0.800 atm for. There is remaining in the container after the reaction is complete.
(c) Use the partial pressures of and to calculate the partial pressure of the consumed in the combustion.
(d) On the basis of your answers above, write the balanced chemical equation for the combustion reaction and determine the formula of the hydrocarbon.
(e) Calculate the mass of the hydrocarbon that was combusted.
(f) As the vessel cools to room temperature, droplets of liquid water form on the inside walls of the container. Predict whether the pH of the water in the vessel is less than 7, equal to 7, or greater than 7. Explain your prediction.
3. A sample of is placed in an insulated container, where it decomposes into ethene and ammonia according to the reaction represented above.
(a) Using the data in the table above, calculate the value, in of the standard entropy change, ΔS°,for the reaction at 298 K.
(b) Using the data in the table below, calculate the value, in , of the standard enthalpy change, ΔH°,for the reaction at 298 K.
(c) Based on your answer to part (b), predict whether the temperature of the contents of the insulated container will increase, decrease, or remain the same as the reaction proceeds. Justify your prediction.
SECTION II, Part B部分