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(Solution) - Find the pH and composition of saturated CaSO4 in water

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Find the pH and composition of saturated CaSO4 in water, considering Reactions 7-23 to 7 27 and using Davies activity coefficients. Suggested procedure: Use activity coefficients in all equilibrium expressions. Let the initial ionic strength be 0. (i) In the mass balance 7-29, express [CaOH+] in terms of [Ca2+] and [H+]. Express [HSO-4] in terms of [SO42-] and [OH-]. (ii) From the Ksp equilibrium expression, express [SO42-] in terms of [Ca2+]. From the Kw equilibrium, express [OH-] in terms of [H+]. The mass balance now contains only [Ca2+], [H+], and equilibrium constants. (iii) Solve for [Ca2+] in terms of [H+]. (iv) Set up a spreadsheet in which pH is the input. Guess an initial value pH = 7.00. Compute . (v) From [H+], compute [OH+], [Ca2+], [CaOH+], [SO42-], and [HSO4-]. (vi) Compute the net charge of the solution from charge balance 7-28. (vii) Use Solver or Goal Seek to vary pH until the net charge is 0 (1 × 10-18M). (viii) Compute the ionic strength. (ix) With the new ionic strength, use Solver again to find the pH that reduces the net charge to < 1 × 10-18 M. Perform several iterations until ionic strength no longer changes.


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This question was answered on: Jul 11, 2017

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