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Ferric chloride ($\ce{FeCl3}$) is a chemical used in water treatment. If ferric chloride is added to a water sample containing calcium bicarbonate, would the $\mathrm{pH}$ of the solution decrease or increase?

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If I'd state that the pH drops, would you know why?

If you dissolve $\ce{FeCl3}$ in water, a hexaqua complex is formed. These complexes are known as cation acids:

$$\ce{[Fe(H2O)6]^{3+} + H2O -> [Fe(OH)(H2O)5]^{3+} + H3O+}$$

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  • $\begingroup$ But where does the Cl3 went in the case? $\endgroup$ – Loïc Faure-Lacroix Apr 2 '18 at 19:04
  • $\begingroup$ @LoïcFaure-Lacroix There's no $\ce{Cl3}$ but three chloride ions $\ce{Cl-}$ acting as counter ions to the aquo complex. They just float around, happily solvated by water molecules. $\endgroup$ – Klaus-Dieter Warzecha Apr 3 '18 at 4:41

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