CHM 475
Intro. to Chemical Oceanography
Dr. S.A. Skrabal
NAME: SOLUTIONS
Exam I
4 February 2004
Instructions: Show work on calculations when required. Useful information is located on the
back pages of the exam. The value of each question is shown in parentheses after each
question. Good luck!
NOTE: TABLE OF MAJOR SEAWATER IONS WAS INCLUDED ON THE EXAM AS WELL
AS TABLE OF ACTIVITY COEFFICIENTS FROM THE TEXTBOOK.
1. Consider the deep water column in the Atlantic Ocean off Wilmington, NC. In separate
figures, draw the density and temperature distributions of the upper 1500 m in (a) January and
(b) August. Be sure to show and label both the seasonal and permanent thermoclines and the
pycnocline in each figure. Be sure to show reasonable values for both temperature and
density. (12)
2. Given the concentration of the major ions in salinity 35 seawater (see Table 1 at the end of
the exam), calculate the concentration of Br- in a sample of salinity 12.5. Express the Brconcentration in units of (a) g kg-1 and (b) mol kg-1. (12)
(a) (6.73 x 10-2 g Br-/kg sw) / 35.00 = x / 12.5; x = 2.40 x 10-2 g Br-/kg sw
(b) (8.44 x 10-4 mol Br-/kg sw) = x / 12.5; x = 3.01 x 10-4 mol Br-/kg sw
3. A solution of salts is prepared for an experiment. It consists of 3.5 x 10-2 M NaCl (sodium
chloride), 7.5 x 10-2 M KNO3 (potassium nitrate), 2.00 x 10-3 M NaHCO3 (sodium bicarbonate),
and 2.5 x 10-2 M MgCl2 (magnesium chloride). All the salts are completely dissociated in
solution. Calculate the ionic strength of this solution (in M). (13)
μ = 0.5 ( ci zi2) = (3.5 x 10-2 M)(1)2 + (3.5 x 10-2 M)(-1)2 + (7.5 x 10-2 M)(1)2 + (7.5 x 10-2 M)(1)2
+ (2.00 x 10-3 M)(12) + (2.00 x 10-3 M)(-1)2 + (2.5 x 10-2 M)(22) + (2)(2.5 x 10-2 M)(-1)2 =
0.187 M or 0.19 M rounded correctly
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4. Certain regions in the equatorial zone, in which the Northern Hemisphere ocean gyres and
Southern Hemisphere ocean gyres interact, are well-known upwelling areas. Explain why
upwelling occurs in these regions, preferably using a diagram. Be as specific as possible. (12)
Clockwise oceanic
gyre in N.
Hemisphere
(prevailing winds and
Coriolis effect)
EQUATOR
Due to Coriolis effect, mean water transport is to the right in
N. Hemisphere and to the left in S. Hemisphere.
Divergence leads to upwelling to replace water from water
masses moving away from one another.
Counterclockwise
gyre in S.
Hemisphere
(prevailing winds
and Coriolis effect)
5. Calculate Kcond for the formation of the KSO4- ion pair in typical salinity 35 seawater given
that the concentration of this ion pair is 1.8 x 10 -4 mol kg-1 and that 38% of the total SO42- and
98% of the total K+ exist as free ions. The Kcond expression must be written correctly to answer
this question. See Table 1 for total ion concentrations. (12)
Reaction: K+ + SO42-  KSO4+
K cond
[ KSO4 ]
 
[ K ] free [ SO42 ] free
K cond

1.8 x10 4 mol kg 1
(0.98)(1.021x10 2 mol kg 1 )(0.38)( 2.823x10 2 mol kg 1 )
 1.7 kg mol 1
(Note: Don’t worry about the weird units of kg mol-1. The units of Kcond will vary depending on
exactly what appears in the Kcond expression.)
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6. Calculate the solubility (in mol L-1) of CaSO4 (calcium sulfate, also known as the mineral
anhydrite) in salinity 35 seawater. The thermodynamic solubility product constant for CaSO 4 is
2.4 x 10-5. Use activity coefficients shown in Table 2 at the end of the test. The reaction is:
(12)
CaSO4 (s)  Ca2+ (aq) + SO42- (aq).
K sp
 {Ca 2 }{SO42 }  [Ca 2 ] Ca 2  [ SO42 ] SO2 
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For every mol of CaSO4 that dissolves, 1 mol each of Ca2+ and SO42- are produced. So at
equilibrium, [Ca2+] = [SO42-] = x. From Table 2, γ for Ca2+ = 0.256 and γ for SO42- = 0.219.
Ksp = 2.4 x 10-5 = (0.256) (x) (0.219)(x) = (5.606 x 10-2) x2
x2 = 4.281 x 10-4; x = 2.1 x 10-2 mol kg-1 = solubility
(2 sig. figs. because 2 in Ksp.)
7. A semi-enclosed basin in a part of the ocean contains a volume of 2.5 x 1017 L and receives
an average riverine input of 4.7 x 1012 L y-1. The average Fe concentration in the basin and in
the riverine input is 8.8 x 10-9 M and 5.0 x 10-6 M, respectively. Assuming the basin is at
steady-state, calculate the residence time (in y) of Fe in the enclosed basin. (12)
τ = A/(dA/dt)
A = (2.5 x 1017 L)(8.8 x 10-9 mol L-1) = 2.2 x 109 mol Fe
dA/dt = (4.7 x 1012 L y-1)(5.0 x 10-6 mol L-1) = 2.35 x 107 mol Fe y-1
τ = 2.2 x 109 mol Fe/2.35 x 107 mol Fe y-1 = 93.6 or 94 y (2 sig. figs.)
Short τ indicates relatively high reactivity of Fe (it doesn’t stay in solution very long).
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8. Short answers and definitions (list or describe in not more than one sentence)
(a) Why is calcium sometimes slightly non-conservative in surface ocean waters? (3)
Biological uptake (CaCO3 shell formation)
(b) What are the three general types of ionic interactions in seawater? (3)
1) Hydrated ions—no interaction of hydration spheres
2) Ion pair formation—interaction of hydration spheres
3) Complexes—covalent interactions—sharing of electrons
(c) Why is the concentration scale mol kg-1 preferable to mol L-1 when describing the
concentration of solutes in the ocean? (3)
Molarity (mol L-1) is a temperature- and pressure-dependent scales whereas mol kg-1 is not
dependent on T or P.
(d) List two processes which can conservatively affect the concentrations of major solutes in
seawater. (3)
Freezing, evaporation, precipitation (rain/snow), advection, riverine input, melting, diffusion
(e) List two processes which can non-conservatively affect the concentrations of major solutes
in seawater. (3)
Chemical precipitation (formation of solids), freezing (if solutes are incorporated into ice),
biological uptake, various chemical reactions, hydrothermal inputs
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Useful information
I
 


 0.5  ci z i2
A
dA
dt
Table 1. Concentrations of major constituents in salinity 35.000 seawater.
Table 2. Single ion activity coefficients in salinity 35.000 seawater.
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