Both Mg(H_2O)_4 (OH)_2 and A1 (H_2O)_3 (0H)_3 can precipitate from solutions whi

Question

Both Mg(H_2O)_4 (OH)_2 and A1 (H_2O)_3 (0H)_3 can precipitate from solutions which contain the individual cations when 1 M NH_3 is added. If you knew that your unknown solution was ACIDIC and you attempted to test it for the Mg(II) ion by adding 1 M NH_3 to precipitate Mg(H_2O)_4 (OH)_2, then even before any of the NH_3 reacted with the Mg(H_2O)_6^2+ ion, what must happen to the [NH_4^+] as a consequence of adding NH_3 to an acid? Explain. Discuss how this change in [NH_4^+] would affect the precipitation of magnesium ion as Mg(H_2O)_4(OH)_2 if Mg(H_2O)_6^2+ had been present?

Explanation / Answer

a)

addition of NH3 to an acidic solution will simply:

NH3 + H3O+ --> NH4+ + H2O

so NH4+ will remain in solution

this will be basic, since H3O+ ions are losing H+ ions to form H2O

b)

if NH4+ increases, then there is more Oh in solution and H2O

so the shift should go toward more produciton of precipitate

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