4. Both Mg (H204(OH)2 and A1 (H20)3 (OH)3 can precipitate from solutions which c

Question

4. Both Mg (H204(OH)2 and A1 (H20)3 (OH)3 can precipitate from solutions which con tain the individual cations when 1 M NH3 is added a) If you knew that your unknown solution was ACIDIC and you attempted to test it for the Mg (II) ion by adding 1 M NH3 to precipitate Mg (H20)4(0H)2, then even before any of the NH3 reacted with the Mg (H20)6 on, what must happen to the ENH4 as a consequence of adding NH3 to an acid? Explain. b) Discuss how this change in [NH4+] would affect the precipitation of magnesium ion as Mg (H204 (OH)2 if Mg (H206+ had been present?

Explanation / Answer

4. (a) Magnesium Hydroxide is sparingly soluble in water but it is readily soluble in the presence ammonium salt. If the solution is sufficiently acidic to neutralize added NH3 and converted into its ammonium salt. Then no precipitation will be observed. If excess base is added then precipapitate may appear depending on the concentration of Mg2+  and pH of your test solution. Ksp for magnesium Hydroxide is 3.4 * 10 -11 . Say for example for the precipitation of 0.1(M) Mg2+ as Mg(OH)2  pH value should be around 9 otherwise no precipitate will be observed. If large amount of NH4+ is present then no precipitation will be observed.

(b) In presence of NH4+ Magnesium Hydroxide will become soluble as Mg(H2O)62+ . All precipitated magnesium hydroxide will disappear resulting a clear solution.

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