Chapters 18 and 19 Practice Test

Multiple Choice (Choose the best answer.)

1. The molar solubility of Cr(OH)₂ is 1.26 x 10^-8. What is its K_sp?

   1.3 x 10 ^-8

   6.8 x 10 ^-31

   1.6 x 10 ^-16

   72.5 x 10 ^-32

   None of these

2. Given the information in problem 9, what is the pH of Cr(OH)₂?

   6.11

   6.40

   7.60

   7.89

   None of these

3. How would the solubility of Cr(OH)₂ be affected if it were dissolved in an acidic solution?

   The solubility would increase.

   The solubility would remain unchanged because the temperature remains the same.

   The solubility would remain unchanged because the concentration of Cr²⁺ is unchanged.

   The solubility would decrease.

   It is impossible to predict without knowing the concentration of the acid solution.

4. Which of the following sulfates has the greatest solubility in terms of g/L?

   CaSO₄, K_sp = 2.4 x 10 ^-5

   SrSO₄, K_sp = 7.6 x 10 ^-7

   PbSO₄, K_sp = 1.3 x 10 ^-8

   BaSO₄, K_sp = 1.5 x 10 ^-9

   RaSO₄, K_sp = 4.0 x 10 ^-11

5. At room temperature, the K_sp for Ag₂CrO₄ is 1.1 x 10 ^-12. Its molar solubility is:

   1.03 x 10 ^-4

   8.19 x 10 ^-5

   6.5 x 10 ^-5

   1.05 x 10 ^-6

   None of these

6. What is the the concentration of Ag⁺ when Ag₂CrO₄ just begins to precipitate from a 0.15 M solution of Na₂CrO₄? Use the K_sp from problem 13.

   7.3 x 10 ^-12

   1.4 x 10 ^-6

   2.7 x 10 ^-6

   5.4 x 10 ^-6

   None of these

7. Given the following enthalpies of formation:

   CO (g) = -110 KJ/mol	CO2 (g) = -394 KJ/mol
   Fe(CO)5 (g) = -733 KJ/mol	Fe2O3 (s) = -824 KJ/mol

   calculate the standard enthalpy change for the reaction:

   2 Fe(CO)₅ (g) + 3 CO₂ (g) Fe₂O₃ (s) + 13 CO (g)

   - 4902 KJ

   - 394 KJ

   193 KJ

   394 KJ

   4902 KJ

8. The first law of thermodynamics states that the internal energy of a system is equal to any heat absorbed or lost by the system plus the work done on or by the system (U = q + w). If heat flows from the system to the surroundings and the volume of the system decreases, the signs of "q" and "w" are:a) q is (+) and w is (+)

   q is (+) and w is (+)

   q is (+) and w is (-)

   q is (-) and w is (+)

   q is (-) and w is (-)

   None of these is correct.

9. Which of the following reactions would likely have a (+) S value?

   CS₂ (g) CS₂ (l)

   CaCO₃ (s) CaO (s) + CO₂ (g)

   2 Hg (l) + O₂ (g) 2 HgO (s)

   More than one have a (+) S value.

   None of the above have a (+) S value.

10. Assume that the reaction Br₂ (l) Br₂ (g) is at equilibrium. If the heat of vaporization of Br₂ (l) is 30.7 KJ/mol and the S = 93.2 J/(mol K), estimate the boiling temperature of bromine.

    329 K

    329 ^oC

    3.04 K

    3.04 ^oC

    None of these.

11. Which of the following statements about the free energy change of a reaction (G) is NOT correct:

    G is an indication of the spontaneity of a reaction

    G is the quantity of "useful" work available in a chemical reaction.

    G is influenced by temperature.

    G must be a positive value if a reaction is exothermic and disorder increases.

    G is equal to H - T S

12. The data below refer to the reaction: Cl₂ (g) + F₂ (g) 2 ClF (g)

    Compound	Hf o(KJ/mol)	Sf o(J/ (mol K))
    Cl2 (g)	0	223
    F2 (g)	0	203
    ClF (g)	-54.6	218

13. What is the value of the free energy change for this reaction at 25 ^oC?

    54.5 KJ

    -109.0 KJ

    106.0 KJ

    -112.0 KJ

    -57.5 KJ

Problem Solving (Show all work. Pay attention to significant figures.)

1)	Ca(OH)2 has a Ksp value of 6.5 x 10 -6. Calculate the solubility in grams per liter.

 

2)	A solution is 0.010 M NaCl and 0.010 M NaBr. AgNO3 is then added drop by drop. (Ksp of AgCl is 1.8 x 10-10 while that of AgBr is 5.0 x 10-13.)
a)	Which precipitates first . . .AgCl or AgBr?
b)	If you decide that AgCl precipitates first, what percentage is of Cl- is left when AgBr starts to form? If you decide that AgBr precipitates first, what percentage of Br- is left when AgCl starts to form?

 

 

 

 

3) For the reaction: CO₂(g) + H₂ (g) CO (g) + H₂O (g) the following data are available:

Compound	Hf o(KJ/mol)	Sf o(J/ (mol K))	Gf o(KJ/mol)
CO2(g)	-393.5	213.6	-394.4
H2 (g)	0	130.6	0
CO (g)	-110.5	197.5	-137.2
H2O (g)	-241.8	188.7	-228.6

a) Calculate the entropy change for the reaction.

b) Calculate the enthalpy change for the reaction

c) Calculate the free energy change for the reaction at 25 ^oC and indicate whether the reaction is spontaneous.

d) Calculate the free energy change for the reaction at 100 ^oC and compare the value with that found in c).

e) Calculate the temperature at which the reaction becomes reversible and comment on its value.

 

 

Answers to Problem-Solving:

1)	0.87 g/L
2)	AgBr ppts 1st, 0.27 % of Br- remains when AgCl ppts
3)	a) 42 J/mol K -- b) 41.2 KJ/mol -- c) 29 KJ/mol (Not Spontaneous) -- d) 26 KJ/mol (Still Not Spontaneous - e) 980 K (At large Ts, rxn becomes spontaneous)

 

 

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