CHEM112 - FINAL EXAM Spring00

Choose the BEST answer to the following 15 questions:

1. The freezing point of a solution made by dissolving 0.250 g of a molecular compound in 200.0 grams of water is -0.056° C. Calculate the molecular weight (molar mass) of the compound. The compound does not ionize in water, and K_f for water is 1.86° C/molal.

a) 30. g/mol
b) 60. g/mol
c) 660 g/mol
d) 140 g/mol
e) 42 g/mol

2. When 0.0100 moles of Pb²⁺(aq) reacts with 0.0200 moles Cl to form PbCl₂(s) in a cup containing 100.0 grams of solution, the temperature rises by 4.73° C. The specific heat of the solution is 4.1 J/g° C. What is the DH° of the reaction in units of kilojoules/mol-reaction?

a) -194 kJ/mol
b) -1.94 kJ/mol
c) -388 kJ/niol
d) 97 kJImol
e) -97 kJ/mol

3. Given the table below, what is the DH (in kJ/mol-rxn) for the reaction:

4 HNO₃(l) = 4 NO₂(g) + 2 H₂O(l) + O₂(g)

SUBSTANCE	DH_f(kJ/mol) at 298K
HNO₃(l)	-174
H₂O(l)	-286
NO₂(g)	+34

a) -426 kJ/mol-rxn
b) +260 kJ/mol-rxn
c) -88 kJ/mol-rxn
d) +192 kJ/mol-rxn
e) -144 kJ/mol-rxn

4. From the initial rate information below, determine the rate-law expression for the reaction A + 2B = C + D:

Run #	Initial Rate (M/s)	[A]_o	[B]_o
1	1.4 x 10^-6	0.300	0.400
2	2.8 x 10^-6	0.300	0.800
3	2.8 x 10^-6	0.600	0.200

a) Rate = k[A][B]
b) Rate = k[A][B]²
c) Rate = k[A][B]²[C]
d) Rate = k[A]²[B]
e) Rate = k[B]

5. The reaction A---> products is a first order decomposition with a rate constant of 2.0 x 10^-3 s^-1. If there is initially 5.00 grams of A, what will be the mass of A after the reaction has been in progress for 120 seconds?

a) 0.787 g
b) 3.93 g
c) 2.74 g
d) 2.50 g
e) 4.85 g

6. Given the following two reactions and their DH values:

Hg(l) + Cl₂(g) = HgCl₂(s); DH = -230. kJ/mol
Hg₂Cl₂(s) + Cl₂(g) = 2 HgCl₂(s); DH = -300. kJ/mol

What is the DH for the reaction 2 Hg(l) + Cl₂ = Hg₂Cl₂(s)?

a) -560 kJ/rnol-rxn
b) -70 kJ/rnol-rxn
c) -160 kJ/rnol-rxn
d)+70 kJ/rnol-rxn
e) +160 kJ/rnol-rxn

7. Given the following equilibrium: N₂(g) + 3 H₂(g) <==> 2 NH₃(l). Notice that ammonia is a liquid in this reaction. This reaction has a DH = -92 kJ/mol-rxn. Consider the following three changes that can be made to this system at equilibrium:

I. The pressure is increased at constant temperature.
II. The temperature is increased at constant pressure.
III. The amount of NH₃(l) is decreased (but not removed completely).

Which of the below lists ALL the changes that will result in a greater equilibrium amount of NH₃(l)?

a) I
b) I and II
c) I, II, and III
d) I and III
e) II and III

8. What is the pH of a solution that is 0.25 M in HNO₂(aq) and 0.050 M in NaNO₂(aq)? The K_a for HNO₂ is 4.5 x 10^{- 4}.

a) 2.65
b) 1.97
c) 3.34
d) 4.04
e) 7.00

9. What is the percent ionization of 0.50 M HF(aq)? K_a for HF is 7 2 x 10^{- 4}.

a) 100%
b) 7.2%
c) 3.8%
d) 2.7 %
e) 0.072%

10. According to Lewis theory, an acid is:

a) a proton acceptor
b) a proton donor
c) an electron pair acceptor
d) an electron pair donor
e) a compound with an electron pair

11. Consider the reaction Cl₂(g) + 3 F₂(g) <== > 2 ClF₃(g). Suppose that initially a mixture has the concentration [Cl₂] = 3.00 M and [F₂] = 4.00 M and no ClF₃. At equilibrium, the concentration of ClF₃ is 2.00 M. What is the equilibrium constant, K_c?

a) 0.50
b) 1.00
c) 0.0208
d) 2.0
e) 4.0

12. Consider the reaction H₂(g) + Cl₂(g) <==> 2 HCI(g) which has an equilibrium constant at some temperature of 40. If initially [H₂] = 0.50 M and [Cl₂] = 0.50M, and there is no HCl, what is the concentration of HCl at equilibrium?

a) 1.0 M
b) 0.34 M
c) 0.76M
d) 0.15 M
e) 0.50 M

13. Given the following two half cell potentials:

V²⁺(aq) + 2e^- = V(s); E° = -l.18 v
Ag⁺ + e^- = Ag(s); E° = +0.86

What is the E°_cell for a standard galvanic cell made from these two half-cells?

a) 0.32 v
b) 1.09 v
c) 3.22 v
d) 1.68 v
e) 2.04 v

14. In the galvanic cell described in Problem #13, what will be the reaction occurring at the cathode?

a) 2 Ag⁺ + V = 2 Ag + V²⁺
b) V(s) = V²⁺ + 2e^-
c) Ag⁺ + e^- = Ag
d) Ag = Ag⁺ + e^-
e) V²⁺ + 2e^- = V(s)

15. Suppose that gold is being plated onto another metal in a electrolytic cell. The half-cell reaction producing the Au(s) is AuCl₄^- = Au(s) + 4 Cl^- + 3e^-. If a 0.30 amp current runs for 15.00 minutes, what mass of Au(s) will be plated, assuming all the electrons are used in the reduction of AuCl₄^-? The Faraday constant is 96,485 coul/mol e^-.

a) 0.184 g Au
b) 0.551 g Au
c) 1.84 g Au
d) 0.613 g Au
e) 0.400 g Au

KEY

1)e 2)a 3)b 4)d 5)b 6)c 7)a 8)a 9)c 10)c 11)d 12)c 13)e 14)c 15)a