September 26, 2023, 12:08:39 PM
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### Topic: pH Calculation Question  (Read 816 times)

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#### jgr

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• Mole Snacks: +0/-0 ##### pH Calculation Question
« on: July 29, 2023, 03:48:49 AM »
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 [O2-]=0.00213...M. What do I do from there?

Appreciate your help #### Hunter2

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« Reply #1 on: July 29, 2023, 05:25:52 AM »
What is the Definition of pH?
The MgO has converted to what.

#### jgr

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« Reply #2 on: July 29, 2023, 07:08:42 AM »
I assume the question implies that when Mg dissolves in water, the O2- ion would function somewhat as a base.

#### Hunter2

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« Reply #3 on: July 29, 2023, 07:10:07 AM »
No you will have a reaction with water to get the right compound.

#### Aldebaran

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« Reply #4 on: July 29, 2023, 08:01:17 AM »
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.

#### Barbara Lewis

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« Reply #5 on: August 14, 2023, 04:21:08 AM »
Here is what I think. Given:
Solubility of MgO = 0.0086 g/100 ml

MgO dissociates into Mg^2+ and O^2- ions when dissolved in water.

Solubility of MgO = [Mg^2+] = [O^2-]

0.0086 g MgO / (40.3044 g/mol MgO) = 0.000213 mol/L = 0.000213 M (concentration of O^2-)

To find pH:
pOH = -log(0.000213) = 3.67
pH = 14 - pOH = 14 - 3.67 = 10.33

Therefore, the pH of a saturated solution of MgO is 10.33.

The key steps are using the solubility to find the concentration of O^2- ions, then calculating pOH from that concentration and finally pH = 14 - pOH.

#### Hunter2

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« Reply #6 on: August 14, 2023, 04:36:37 AM »
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

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« Reply #7 on: August 14, 2023, 01:02:46 PM »
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

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« Reply #8 on: September 18, 2023, 09:40:08 AM »
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.