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K sp Fe(OH) 2 ¼ 10 14 ; Fe(OH) 3 ¼ 10 36 ; Cr(OH) 3 ¼ 10 33
What does this tell you about the solubility of these ions?
Does this agree with their observed behaviour in soil?
(v) A soil with a bulk density of 1.2 g cm 3 contains 1% FeS 2 . How
much CaCO 3 is required per hectare to neutralize the acid
produced to a depth of 20 cm? (Assume that the FeS 2 oxidizes
completely. Fe ¼ 56; S ¼ 32; Ca ¼ 40; C ¼ 12; and O ¼ 16.).
(Hint: look at Worked example 5.1 and combine Equations
(5.16) and (5.17) to get an overall reaction for the oxidation of
pyrite.)
(vi) Iron oxides in soil undergo reduction to produce Fe 21 ions-
Fe(OH) 3 þ 3H 1 þ e
Fe 21 þ 3H 2 O E 0 ¼ 1.057 V
What is the concentration of Fe 21 ions in solution in a soil of
pH 7 at 100 and 0 mV?
(vii) 1.00 g samples of a soil were shaken with 50 cm 3 of different
solutions of known zinc concentration. After 1 h, the soil
suspensions were filtered and the concentration of zinc in the
filtrate measured, with the following results:
"
Initial concentration
(mg Zn L 1 )
Final concentration
(mg Zn L 1 )
7.5
1
10
2
20
5
30
10
45
21
60
34
In each case, calculate the weight of zinc held by 1 g of soil. Plot
the graph of weight of Zn held per gram (y-axis) against the final
concentration of Zn.
Fit the data to the Langmuir equation and plot the graph.
Obtain values for x m and K.
(viii) The equilibrium between Fe 31 ions in soil solution and iron
oxides in soil has been described by the equation
Fe(OH) 3 soil þ 3H 1
Fe 31 þ 3H 2 O log K ¼ 2.70
"
 
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