Clearly Show the Method Used and Steps Involved in Arriving at Your Answers

AP EXAM PRACTICE - FREE RESPONSE

due after break

Clearly show the method used and steps involved in arriving at your answers. It is to your advantage to do this, because you may earn partial credit if you do and you will receive little or no credit if you do not. Attention should be paid to significant figures.

You may use a calculator to answer the following questions.

NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)

1. In aqueous solution, ammonia reacts as represented above. In 0.0180 M NH3(aq) at 25 °C, the hydroxide ion concentration [OH-] is 5.60 x10-4 M. In answering the following, assume that temperature is constant at 25 °C and that volumes are additive.

(a) Write the equilibrium-constant expression for the reaction represented above.

(b) Determine the pH of 0.0180 M NH3(aq).

(c) Determine the value of the base ionization constant, Kb, for NH3(aq).

(d) Determine the percent ionization of NH3 in 0.0180 M NH3(aq).

2. A strip of Ni metal is placed into a 1.0 mol/L solution of Ni(NO3)2 and a strip of Ag metal into a 1.0 mol/L solution of AgNO3. An electrochemical cell is created when the two solutions are connected with a salt bridge and the two metal strips are connected by wires to a voltmeter.

(a) Write a balanced equation for the overall reaction that occurs and calculate the cell potential, E°.

(b)

(c) Calculate the value of the standard free energy change, G°, for the cell reaction.

(d) Find K at 25 °C.

(e) Find the cell potential, E, at 25 °C for the cell if the initial concentration of Ni(NO3)2 is 0.100 M and the intial concentration of AgNO3 is 1.20 M.

3. Consider the following solutions: 0.10 M NaF

0.10 M MgCl2

0.10 M C2H5OH

0.10 M CH3COOH

Answer the following questions, which refer to 100 mL samples of each of the solutions mentioned above. Each sample is in a sealed flask at 25 °C.

(a) Which solution has the lowest electrical conductivity? Explain.

(b) Which solution has the lowest freezing point? Explain.

(c) Above which solution is the pressure of water vapor the greatest? Explain.

(d) Which solution has the highest pH? Explain.

4. Answer the following questions using the principles of chemical bonding and molecular structure.

(a) Consider the carbon dioxide molecule, CO2, and the carbonate ion, CO32-.

(i) Draw the complete Lewis electron-dot structure for each species.

(ii) Account for the fact that the carbon-oxygen bond length in CO32- is greater than the carbon-oxygen bond length in CO2.

(b) Consider the molecules CF4 and SF4.

(i) Draw the complete Lewis electron-dot structure for each molecule.

(ii) In terms of geometry, account for the fact that the CF4 molecule is nonpolar, wheras the SF4 molecule is polar.