Chem 123                                               Sample Exam #3 (For Multiple Choice Version, see Sample Exam 3B)

Dr. Drisko, Instructor

1. Oxygen can be made by reacting sodium peroxide and water:

                2Na2O2(s)       +     2H2O(l) ®      4NaOH(s)      +      O2(g)

DH° = -109.0 kJ DG° = -148.4 kJ              at 25°C

                (a) Calculate DS° for this process:

                (b) Calculate S° for Na2O2

2. Show by calculation whether the following reaction

                PbBr2(s) Û       Pb+2(aq)        +    2 Br-1(aq)

is spontaneous at 25°C

                a. when [Pb+2] = [Br-1] = 1.0 M

                b. when [Pb+2] = [Br-1] = 0.0050 M

3.   Consider a voltaic cell in which the following reaction takes place

 

            3H2O2(aq)      +    6 H+(aq)   +   2 Au(s)         ®                2Au3+(aq)    +      6H2O(l)

 

a.       Calculate E°

 

 

b.      Write the Nernst equation for the cell

 

c.       Calculate E when [Au3+] = 0.250 M, [H+] = 1.25 M, [H2O2] = 1.50 M

 

 

4. Calculate E° for the following voltaic cell:

 

            Pb(s)        +       2Ag+(aq)           ®       Pb+2(aq)              +          2Ag(s)

 

5. Calculate E°, DG° and K at 25°C for the reaction (run in basic media)

 

NO3-(aq)       +       H2O(l)      +     2Fe(OH)2(s)      ®     2Fe(OH)3(s)    +       NO2-(aq)

 

6. Calculate the solubility (mol/L) of Ag2CrO4 in:

(Ksp for Ag2CrO4 = 1.0 x 10-12)

            a. H2O

 

 

            b. 0.01 M Na2CrO4

7. A solution contains 0.054 M SO42-.  Calcium chloride is added to precipitate calcium sulfate.  At what concentration of Ca+2 does a precipitate first start to form?

 

 

 

8. Calculate the solubility in grams per liter of magnesium fluoride in 0.025 M Mg(NO3)2(aq).