We are asked to predict whether or not a precipitate will form upon mixing 75.0 mL of a NaOH solution with pOH = 2.58 with 125.0 mL of a 0.018 M MgCl2 solution.
For this, we need to compare the reaction quotient (Q) vs. the solubility product constant (Ksp).
Recall that when:
• Q > Ksp: the solution is supersaturated and a precipitate will form. Reactants are favored.
• Q = Ksp: the solution is at equilibrium and no precipitate will form.
• Q < Ksp: the solution is unsaturated and no precipitate will form. Products are favored.
The expected precipitate is Mg(OH)2. The hydroxide ion, OH–, has a charge of –1. Mg then has a charge of +2.
The dissociation of Mg(OH)2 in water is as follows:
Mg(OH)2 (s) ⇌ Mg2+(aq) + OH–(aq)
Predict whether or not a precipitate will form upon mixing 75.0 mL of a NaOH solution with pOH = 2.58 with 125.0 mL of a 0.018 M MgCl2 solution. Identify the precipitate, if any.
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