Chem 112  Exam III  ::::::  Spring 1999:  Day/Whelan

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K = ° C + 273
at 25 ° C Kw = 1.0 x 10-14
pH + pOH = 14
pH = -log [H3O+] 
pOH = -log [OH-] 
pKa = -log Ka
Ka x Kb = Kw
[H3O+] = Ka{[HA]/[A-]}
Strong Acids: HCl, HBr, HI, HNO3, HClO4, H2SO4
qP = DH
DH° = SDH°f (products) - SDH°f (reactants)
DSuniverse = DSsystem + DSsurroundings ³ 0
DS = qreversible / T 
DSsurroundings = – DHsystem / T
DS° = SS° (products) - SS° (reactants)

1.     (5 Points)
The pH of an 0.10M weak monoprotic acid (HA) is found to be 2.7, what is the value of Ka for this acid?

4.1x10-5
4.5x10-2
4.0x10-7
4.0x10-6
4.5x10-3

2.     (4 Points)
The pH of an 0.1 M acetic acid solution is 2.87. The pH of an 0.1M CCl3CO2H solution is expected to be:

greater than 2.87
less than 2.87
equal to 2.87
without knowing its Ka it is impossible to predict.

3.    (4 Points)
Which of the following aqueous solution would have a pH < 7 ?

      • 1.       1M     NH3
      • 2.       1M     NaCl 
      • 3.       1M     NH4Cl
      • 4.       1M     CH3CO2Na 
      • 5.       1M     H2SO4


1 & 41 & 32 & 43 & 51 & 4

4.     (4 Points)
The pH of aqueous iron(III) nitrate is not 7.0. This is best explained by the equation:

NO3-(aq) + H2O(l) « HNO3(aq) + OH-(aq)
HNO3(aq) + H2O(l) « H3O+(aq) + NO3-(aq)
[Fe(H2O)6]3+(aq) + H2O(l) « [Fe(H2O)5(OH)]2+(aq) + H3O+(aq)
Fe3+(aq) + 6H2O(l) « Fe(OH)3(s) + 3H3O+(aq)
[Fe(H2O)6]3+(aq) + H2O(l) « [Fe(H2O)5(H3O+)]2+(aq) + OH-

Question 5 and 6 refer to an aqueous solution of 0.25M carbonic acid.
[ For H2CO3, K1 = 4.4x10-7, K2 = 4.7x10-11]

5.    (5 Points)
The approximate [H3O+] of this solution is:

6.6x10-4
6.9x10-6
4.7x10-11
3.3x10-4
3.4x10-6

6.    (4 Points)
The approximate [CO32-] in this solution is:

4.7x10-11
6.9x10-6
4.4x10-7
3.4x10-6
2.1x10-17

Use the K's given for Question 5 & 6 to Answer Question 7

7.    (4 Points)
The pH of an aqueous solution of NaHCO3 is expected to be:

less than 7 due to: HCO3-(aq) + H2O(l) « CO32-(aq) + H3O+(aq)
equal to 7
greater than 7 due to: HCO3-(aq) + H2O(l) « H2CO3(aq) + OH-(aq)
less than 7 due to: Na+(aq) + 2H2O(l) « NaOH(aq) + H3O+(aq)
greater than 7 due to: Na+(aq) + H2O(l) « NaH(aq) + OH-(aq)

8.    (4 Points)
Which of the following is defined as an electron pair donor?

A Bronsted AcidA Bronsted Base A Lewis AcidA Lewis BaseAn Arrhenius Acid

9.    (4 Points)
The approximate [H3O+] of a solution that is 0.50M hydrofluoric acid and 0.50M sodium fluoride is: 
[ Ka for HF = 7.2x10-4, Kb for F- = 1.4x10-11]

7.2x10-4
1.9x10-3
2.7x10-2 
1.4x10-11
2.6x10-6

10.     (4 Points)
Which of the following solutions when added to an 0.15M solution of NaNO2 would make the solution an effective buffer?

      • 1.     0.07mol NaOH 
      • 2.     0.07mol NH3 
      • 3.     0.14mol HNO2 
      • 4.     0.07mol HNO3


1 only2 only3 only2 & 33 & 4

11.     (4 Points)
The solubility of barium sulfate in pure water is found to be 1.0x10-5M. What is Ksp for BaSO4?

2.0x10-5
3.2x10-3
5.0x10-6
1.0x10-5
1.0x10-10

Questions 12 and 13 refer to an aqueous solution of silver carbonate.
Ksp for Ag2CO3 at 298K, 8.2x10-12.

12.    (4 Points)
The solubility in moles/L of silver carbonate in pure water at 298K is:

2.0x10-4
8.2x10-12
2.9x10-6
5.7x10-6
1.3x10-4

13.    (4 Points)
The addition of solid potassium carbonate would:

increase the solubility of silver carbonate.
decrease the solubility of silver carbonate.
have no effect on the solubility of silver carbonate.
increase the Ksp of silver carbonate.
decrease the Ksp of silver carbonate.

14.    (4 Points)
Sodium fluoride is slowly added to a solution that is 0.010M in: Ba2+, Ca2+, Pb2+ and Mg2+. The first precipitate is expected to be:
[Ksp values: BaF2 1.7x10-6, CaF2 3.9x10-11, PbF2 3.7x10-8, MgF2 6.4x10-9]

BaF2
CaF2
PbF2
MgF2
impossible to determine, sodium fluoride concentration needed.

15.    (5 Points)
For the solution in question 14 what concentration of fluoride ion is need to initiate precipitation of PbF2 .

1.9x10-4
9.6x10-4
1.9x10-3
3.7x10-6
3.8x10-3

16.     (4 Points)
The heat of fusion of Hg(s) is 2.33 kJ/mol at its normal melting point of 234 K. What is the value of DS in J/K for the process Hg(s) ® Hg(l) at 234K, 1 atm?

2.33 9.961005452330

17.    (4 Points)
Predict which one of the following reactions would have DSo < 0.

NH4Cl(s) ® NH3(g) + HCl(g)
2H2O2(l) ® 2H2O(l) + O2(g)
FeO(s) + H2(g) ® Fe(s) + H2O(l)
N2O4(g) ® 2 NO2(g)
I2(s) + H2(g) ® 2 HI(g)

Questions 18 and 19 refer to the following reaction and thermodynamic data at 298K given below the substances:
   CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(l)
DHof (kJ/mol) – 75         – 394    – 286
So (J/mol K) 186    205     214     70


18.    (4 Points)
What is the standard entropy change for the reaction in J/K at 298K?

– 891– 2422428912990

19.    (5 Points)
If one mole of CH4(g) reacts under standard conditions, what is the entropy change for the surroundings in J/K at 298K?

– 891– 2422428912990

Esssay Portion
20.     (5 Points)
The second law of thermodynamics states that the entropy of the universe increases for any spontaneous process, but the combustion of Mg(s) proceeds spontaneously with a decrease in entropy:

Mg(s) + 1/2 O2(g) ® MgO(s) DSo = – 108 J/K at 298K

Is this a violation of the second law? Briefly explain. (Hint: The standard heat of formation of MgO(s) is – 602 kJ/mol at 298K.)
 

Answer:
Questions 2 - 6 are based on a 1L buffer solution containing 0.20M HCHO2 and 0.10M NaCHO2
[Ka for HCHO2 = 1.8x10-4 @ 298K]

21.    (4 Points)
What is the pH of the buffer solution.
 
Answer:

22.    (3 Points)
Upon addition of a small quantity of H3O+, write the reaction that would take place to remove this added acid.
 
Answer:

23.    (4 Points)
Calculate the resultant pH of the buffer upon the addition of 0.01 moles of HCl. You may assume that there is no volume change.
 
Answer:

24.     (2 Points)
Qualitatively (no pH calculation), describe what effect if any the addition of a small quantity of a strong base would have on the pH of the original buffer solution.
 
Answer:

25.    (2 Points)
Quantitatively (no pH calculation), describe what effect if any the addition of 0.3 moles of NaOH would have on the pH of the original buffer solution.
 
Answer: