[jgr]

Hello,

I came across the question:
Magnesium oxide has a solubility of 0.0086g/100ml. What pH will a saturated solution of magnesium oxide have?

I have that [O^2-]=0.00213...M. What do I do from there?

Appreciate your help

[Hunter2]

What is the Definition of pH?
The MgO has converted to what.

[jgr]

I assume the question implies that when Mg dissolves in water, the O^2- ion would function somewhat as a base.

[Hunter2]

No you will have a reaction with water to get the right compound.

[Aldebaran]

If you look at Wikipedia for both magnesium and calcium oxides and hydroxides it will help you answer your original question and provide lots of other interesting information to broaden your chemistry knowledge.
As an extension you might find the information about inverse solubility of calcium hydroxide interesting and consider what the effect might be on pH compared to a solution of magnesium hydroxide.

[Hunter2]

No,there will be no O2- ions.
The oxid reacts with water as mentioned above.
You measure OH- or H+, but no O2-.
Need dissociations constant and solubility product

[Aldebaran]

In spite of previous posts by Hunter 2 and myself you are looking at the wrong ion in solution. Once you’ve figured out the concentration of OH^-1 ions you can use ionic product of water to calculate pH (assuming room temperature).

[ProfOxidizer]

Of course it is impossible to dissociate oxides. Oxides do not dissociate into ions! They can only dissolve in solvent (e.g. water) as a whole particle. In this exercise there is a reaction  between water and MgO:

MgO+H2O->Mg(OH)2

So, we now know that from 40 g of MgO, there is a creation of 58 g of Mg(OH)2. So, from 0.0086 g, it will be created 0.0125 g Mg(OH)2. We assume that this amount is so low that do not influence the calculations concerning solution volume. Therefore, we assume that 0.0125 g of magnesium hydroxide is in 0.1 dm3 of solution (100 g of water occupy 100 cm3 in normal conditions). Thus, the molar concentration of Mg(OH)2 is equal to 0.0022 M. From 1 mole of magnesium hydroxide we get 2 moles of hydroxides ions. In this situation we obtain 0.0044 M of hydroxide ions concentration. pOH is equal to -log(OH). pH is equal to 14-pOH.