[mix_boom]

Hi all, I'm very new to this forum...nice to konow you all!
Last week, I've done an experiment related to "Common Ion Effect on Sparingly Soluble Salts".
What makes me confused is this:
I put 1 mL of MgSO₄ into a reaction tube.
Then, I add 15 drops of NH₃ into the tube.
After shaking for some time, a white precipitation of Mg(OH)₂ is formed.
Then, I add 15 drops of NH₄Cl into the tube. After shaking again, the precipitation seemed to disappear, though there's still a kind of gelatin there.
A) Write all reactions involved in this experiment.
B) How is the concept of common ion effect involved in this experiment?

Thanks in advance...

[Borek]

You have to show your attempts at solving the question to receive help. This is a forum policy.

Start with the reaction equations. What would you expect to happen?

[mix_boom]

Thanks for the warning...

I've been thinking of this reaction:
MgSO_{4 (aq)} + 2NH_{3 (aq)} + 2H₂O_(l) --> Mg(OH)_{2 (s)} + (NH₄)₂SO_{4 (aq)}

But, I'm not sure about this one:
Mg(OH)_{2 (s)} + NH₄Cl_(aq) --> MgCl_{2 (aq)} + NH₄OH_(aq)

Even if it is correct, I can't see the common ion effect here.
My teacher gave me a clue; it is that the common ion effect relates with the increasing amount of ammonium ion and the constant amount of hydroxil ion.

That's my approach...

And, if the disappearance of magnesium hydroxide precipitation take place, can that precipitation disappears completely?

Thank you...

[9-92-6-19]

Your second reaction is incorrect.

You haven't balanced the number of OH^- ions. The proper reaction is as follows:

Mg(OH)₂(aq) + 2NH₄Cl(aq)  MgCl₂(aq) + 2NH₃(aq) + 2H₂O(l)

We also must remember that NH₃ exists in an equilibrium in solution as ammonium hydroxide, so I could have also written 2NH₄OH, instead; this is where the common ion effect comes into play.

The reaction occurs because NH₄Cl has this equiliburm in solution:

NH₄Cl  NH₃ + H⁺ + Cl^-

The formation of H⁺ causes the Mg(OH)₂ to react and form water and Mg²⁺ in solution; however, this reaction is quickly offset by the latter creation of OH^- ions as a result of NH₃.

[AWK]

The formation of H+ causes the Mg(OH)2 to react and form water and Mg2+ in solution

Not exactly.
The addition of NH₄Cl forms ammonia buffer with concentration of OH^- lower by 1 to 2 orders (10 - 100 times). Then more Mg(OH)₂ have to dissolve to fulfill K_sp  requirements.