September 15, 2024, 06:35:38 AM
Forum Rules: Read This Before Posting

### Topic: pH Calculation Question  (Read 2029 times)

0 Members and 1 Guest are viewing this topic.

#### jgr

• Regular Member
• Posts: 13
• 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?

#### Hunter2

• Sr. Member
• Posts: 2267
• Mole Snacks: +185/-49
• Gender:
• Vena Lausa moris pax drux bis totis
##### Re: pH Calculation Question
« Reply #1 on: July 29, 2023, 05:25:52 AM »
What is the Definition of pH?
The MgO has converted to what.

#### jgr

• Regular Member
• Posts: 13
• Mole Snacks: +0/-0
##### Re: pH Calculation Question
« 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

• Sr. Member
• Posts: 2267
• Mole Snacks: +185/-49
• Gender:
• Vena Lausa moris pax drux bis totis
##### Re: pH Calculation Question
« Reply #3 on: July 29, 2023, 07:10:07 AM »
No you will have a reaction with water to get the right compound.

#### Aldebaran

• Full Member
• Posts: 127
• Mole Snacks: +6/-1
##### Re: pH Calculation Question
« 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.

#### Hunter2

• Sr. Member
• Posts: 2267
• Mole Snacks: +185/-49
• Gender:
• Vena Lausa moris pax drux bis totis
##### Re: pH Calculation Question
« Reply #5 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

• Full Member
• Posts: 127
• Mole Snacks: +6/-1
##### Re: pH Calculation Question
« Reply #6 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 youâ€™ve figured out the concentration of OH^-1 ions you can use ionic product of water to calculate pH (assuming room temperature).

#### ProfOxidizer

• New Member
• Posts: 4
• Mole Snacks: +0/-1
##### Re: pH Calculation Question
« Reply #7 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.