Question

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4. Groundwater in Pherric, New Mexico, is very acidic and initially contains 1.800 mg/L of iron as Felt Use pKsp = 38.57 for Fe(OH)3 (a) To what pH must the water be raised in order to precipitate all but 0.30 mg/L of the iron? The temperature of the water is 25°C. (Hint: "All but 0.30 mg/L" means that there 0.30 mg/L left in solution after the Fe(OH)3 precipitates.) Answer: pH = 3.35 (b) If the pH was higher than the answer in part (a), state whether the Fe+ concentration would be higher or lower than 0.30 mg/L and provide justification.

Answer 1

The Precipilation quilibim fo Fe (ou) be wepesented as Fe Cou3() Fe3 304 the sdubiHy Podu fo tne weortion con be onten a spFCfe Con Cre co [eo- actuty af asolid s daken acuty) PsP-logksp -ogsp 28.5 sp SP-Lo 2.69 x to 39 To Precipitate but o30 mgl of fe offe3t ion will enoin Concentralion of tron a Fe3t is 1 mgl meanes that o.30mgl in sdolton and we have the inilal thaefore the cencentaHon of Pe3t should sedce fom l o/o018 glt to o.0003 glu .bngl (tmg 0.0019) theie fore he corcentaton p Po3t shaold be ০০०16 ৭lt - 0.6003 9L 1SX1039l

Conme tin lo melelı we Jet (22 1SKIO 3h of fe3, mol tssgFe 2.6 1 mol lu Puting tnese value n tne solubility Product uatent oeget Sp Fe Co-3 34 Oa) 2.69to -.61o- ToH)3 2.6 I63 12.6 x10 o 3 2.1s to PE poH-tog (oH) lo 2xtJ l0g to] log 2.15 - -o,33 - 10.67 PH t POH 4 PH 4-o. b 3r 33

3) the pmust be ased to 333 Ao Perptate all but o.mg of the fron T the p s higherttan the ansue n pa Ca1thad s highe han 33, he Concentaton Fe3f would be lowe than o.30mgl et us Suppase that the pH ps .33, Po ooold be PoH ।५-5.33 8.6 Os)log foM 8.64 -8.6 2,138x to9 Poting thos values to sdoa yprodut euaton g CFe a 2,138K10] 2.69o39 o Fet2.6x 132.13 023x2 mollL is.bx 109 m mell CO.28 RIO12 o.28K102 molL x ss.8q FeT mol FesxIoco mg/ 19 is.bx tomgl