Chemistry 222
Fall 2015
Exam 2: Chapters 5,6,7
80 Points
Name__________________________________________
Complete two (2) of problems 1-3, problem 4, and three (3) of problems 5-8. CLEARLY mark the
problems you do not want graded. You must show your work to receive credit for problems
requiring math. Report your answers with the appropriate number of significant figures.
Do two of problems 1-3. Clearly mark the problem you do not want graded. (10 pts each)
1. A real solution at equilibrium will likely contain several equilibria. The statement “all
equilibrium conditions must be satisfied simultaneously” is often used in describing
these systems. Clearly describe the chemical significance of this statement, especially as it
pertains to an ion or molecule that appears in more than one equilibrium in the same solution.
2. Three primary factors that play a role in determining activity coefficients of ions. Identify each
and briefly describe the impact of each factor on the activity of an ion.
3. If I prepare a saturated silver chloride (Ksp = 1.8 x 10-10) solution by putting 100 g of AgCl in
10 mL of water and you prepare a saturated silver chloride solution by putting 100 g of AgCl
in 100 mL of water, what is the relative concentration of Ag+ in your solution compared to
mine? Clearly explain your reasoning.
You MUST do problem 4. (15 points)
4. Consider a 0.010 M silver nitrate solution that is saturated with silver carbonate AND silver
chloride. Set up the equations necessary to determine the solubility of silver carbonate,
considering the equilibria below. You must write the charge balance expression and at least
one mass balance. Identify all unknowns and write enough explicit, independent mass
balance, charge balance, and equilibrium expressions so that only algebra remains to solve
for the unknowns. A numerical answer is not necessary.
Ag2CO3 Ksp = 8.1 x 10-12
AgCl
Ksp = 1.8 x 10-10
H2CO3 Ka1 = 4.46 x 10-7, Ka2 = 4.69 x 10-11
H2O
Kw = 1.0 x 10-14
Do three of problems 5-8. Clearly mark the problem you do not want graded. (15 pts each)
5. Clearly describe the case when it is preferable to use calibration by standard additions, rather
than a traditional calibration curve for an analysis. Include an example of how you would run
the experiment and extract an unknown concentration from your data.
.
6. Using activities, calculate the fluoride concentration in a saturated solution of calcium fluoride
in a solution that contains 0.010 F magnesium nitrate and 0.020 F sodium chloride. The Ksp
for calcium fluoride is 3.2 x 10-11, assume that all other salts are soluble. You may ignore the
autoprotolysis of water and any acid-base character of the ions in solution.
7. What is the silver ion concentration in a solution prepared by mixing 50.0 mL of 0.496 M
silver nitrate with 50.0 mL of 0.387 M sodium carbonate? The Ksp of silver carbonate is
8.1 × 10-12. You may ignore autoprotolysis and the acid-base behavior of carbonate ion.
8. In an instrumental method for the determination of mercury in a sample, a calibration curve
was prepared to relate the mercury signal (in Volts) to mercury concentration (in ppm).
Least-squares analysis of the data resulted in the relationship:
Signal = (0.0556 V/ppm)(concentration) + 0.0001 V.
Five blank measurements gave the following results: 0.0003 V, 0.0002 V, 0.0002V,
0.0001 V, 0.0000 V. What is the detection limit for this method?
Blank Space if You Need Extra Room
Possibly Useful Information
KaKb = KW = 1.0 x 10-14
pH = -log [H+]
y = mx + b
SLOD = Sblank + 3sblank
log  
 0.51z 2 

1    305

G = H -TS = -RTlnK
Ix
Is  x


(with  in pm)
x i
kx i

k sf  x f  sf  x f
x
1
2
 c i zi
2 i
 b  b 2  4ac
2a


Standard Signal
Analyte Signal

 F
Standard
Concentrat
ion
Analyte Concentration

