Problem: 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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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)


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Problem Details

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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What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Selective Precipitation concept. If you need more Selective Precipitation practice, you can also practice Selective Precipitation practice problems.

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Based on our data, we think this problem is relevant for Professor Thavarajah's class at TORONTO.