8. Completely soluble nontoxic salts such as Fe,(SO), are often used during wate

Question

8. Completely soluble nontoxic salts such as Fe,(SO), are often used during water purification to remove soluble toxic contaminants, because they form gelatinous hydroxides that encapsulate the contaminants and allow them to be filtered from the water. Calculate the molar solubility in water of Fe(01), at 25°C. ,what is the concentration of hydroxide ion in a saturated solution of Fe(OH),? Calculate the pH of the solution. (c) Discuss whether your result in part (b) is reasonable for a solution of a basic hydroxide Explain what assumptions were made in your calculations and evaluate the validity of the assumptions A simplified equation for the reaction of Fe ions with water is FeTaq1+ 6H,O(l)-Fe(OH),(s) + 3H30.(aq). Calculate the equilibrium constant for this reaction. e) If 100 g of Fe,(SO,), is dissolved i n enough water to make up 1.00 L of aqueous solution and the pH of the solution is raised to 8.00 by addition of NaOH, what mass of solicd Fe(OH), will form? the ability of Fe,(SO,), to remove chloride ions from water, a standard aqueous solution containing 24.72 g of NaCl in 1.000 L of solution was prepared. A 25.00-mL sample of the NaCIl solution was then combined with the mixture described in part (e) and stirred. The Fe(OH), precipitate containing encapsulated chloride ion was removed by filtration, then dissolved in acid. An aqueous solution of AgNO, was then added to the resulting solution and the solid AgCI formed filtered and dried. The mass of AgCl was 0.604 g. What percentage of the chloride ion in the 25.00-mL sample had been removed from the solution?

Explanation / Answer

(a) The solubility product of Fe(OH)3, Ksp = s*(3s)3 = 27*s4

The actual Ksp of Fe(OH)3 = 6 x 10-38

i.e. 27*s4 = 6 x 10-38

i.e. s = molar solubility of Fe(OH)3 = 2.17*10-10 M

(b) The concentration of hydroxide in a saturated solution of Fe(OH)3 = 3*2.17*10-10 = 6.51*10-10 M

pOH = -Log[OH-] = -Log(6.51*10-10) = 9.19

i.e. pH = 14 - pOH = 14 - 9.19 = 4.81

i.e. [H+] = [H3O+] = 10-4.81

(c) The result in part is not reasonable for a solution of a basic hydroxide.

Reason: pH should be greater than 7 for a basic solution, here pH < 7. Hence, it is not reasonable for a solution of basic hydroxide.

(d) The equilibrium constant (K) = [H3O+]3 / [Fe3+] = 10-4.81 / (2.17*10-10) = 7.13*104

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