Chemistry Help Forum Questions related to precipitation of Fe3+ and Al3+ by Ca(OH)2

 Dec 9th 2018, 05:55 AM #1 cuigm371 Junior Member   Join Date: Dec 2018 Location: canada Posts: 1 Questions related to precipitation of Fe3+ and Al3+ by Ca(OH)2 I have a problem to resolve for my upcoming exam. A liquid containing 1mol/L of cobalt (Co2+) as cobalt sulfate is contaminated with iron and aluminum and should be purified by removing these impurities by adding slaked lime (Ca(OH)2). The concentration of individual impurities in the treated solution is not to exceed 10^-3 mol/L. 1. What pH would you recommend for this operation? 2. Following the lime addition, what would be the composition of the solution at the chosen pH? 3. How much lime needs to be added per L of solution? will lime dissolve fully? 4. How much gypsum per kg of cobalt in the feed will be produced in each case? Known: Log Ksp for the reactions: Fe3+ + 3OH- = Fe(OH)3 38.8 Al3+ + 3OH- = Al(OH)3 33.5 Co2+ + 2OH- = Co(OH)2 14.9 Ca2+ + 2OH- = Ca(OH)2 5.26 Many thanks! My attempt: a. from: Co2+ + 2OH- = Co(OH)2 14.9 K=[Co2+][OH-]^2/[Co(OH2)] = 1 * [OH-]^2 = 10^(-14.9) so [OH-] = 10^(-7.45) so pH= 6.55 Al3+ + 3OH- = Al(OH)3 33.5 K=[Al3+][OH-]^3/[Al(OH3)] = 10^(-3) * [OH-]^3 = 10^(-33.5) so [OH-] = 10^(-10.2) so pH= 3.8 No need to check Fe since it will be lower than that for Al. therefore 3.8

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