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Equilibrium
Question
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Give reason:  The precipitation of Mg(OH)2 is prevented by the addition of NH4Cl prior to the addition of NH4OH. But its precipitation by NaOH is not prevented by the prior addition of NaCl.

Answer

It can be explained by common ion effect.

Due to common ion effect, NH4ions suppress the ionisation of NH 4OH. The low concentration of OH– ions is not sufficient for the ionic product [Mg2+][OH-]2 to exceed the solubility product of Mg(OH–)2. Hence it is not precipitated.
On the other hand, NaOH is a strong base and ionises almost completely. Its dissociation is not much affected by the common ion (Na+) from NaCl. Thus the ionic product [Mg2+][OH–]2 exceeds the solubility product value of Mg(OH)2. Hence Mg(OH)2 gets precipitated.

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