How to prove something is partially soluble?

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Nyx:)
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#1
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#1
So I'm revising chemistry and I have a couple questions I'm stuck on.
Without any visual indicators, how do you prove that a sample was partially soluble after putting it into the solvent(water)?

Why does every magnesium ion have two hydroxide ions in Mg(OH)2?

Your help would be greatly appreciated!
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bl0bf1sh
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#2
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#2
what is the charge of a magnesium ion? what is the charge of a hydroxide ion?
in order for the Mg(OH)2 compound to have no overall charge, what must the ratio of magnesium ions to hydroxide ions be?
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Trinculo
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#3
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#3
Presumably weigh its dry mass beforehand, put it into the solvent, filter the solvent out and then weigh the dry mass afterward.
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scimus63
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#4
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#4
if its soluble the solution formed should be clear, if its cloudy its partly soluble I guess!

Mg ion have a +2 charge, hydroxide ions have a - charge. You need 2 hydroxide ions to balance off the charges.
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Nyx:)
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#5
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#5
Thank you everyone! I understand now, I was just making it more complicated in my head.

There's one last one if you can help:
Mg(OH)2 - - > Mg2+ + 2 OH-

What would happen to the position of the equilibrium and amount of mg(oh)2 if you add HCl?

So I think the 2OH- and the 2H+ would make H2O, and the Mg2+ and 2Cl- would make MgCl2. So this would essentially decrease the amount of Mg(OH) 2 solid. So the equilibrium would move to the right, to the products side?

Is this correct?
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scimus63
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#6
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#6
Hi,
Your reasoning is OK but I think neutralisation reactions generally go to completion, they are not equilibrium reactions. If you add HCl to magnesium hydroxide I think the reaction will go pretty much 100% to products, magnesium chloride and water.

You also do not include solids in equilibrium constants- you cannot measure the concentration of a solid, like an ion or a gas in an equilibrium expression.

If this was an equilibrium reaction then what you say is good!

try here it might help:
https://science-revision.co.uk/A_Lev...constants.html

are you in y12?
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scimus63
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#7
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sorry but I have just read your original question about partly soluble- you could filter it! If its insoluble you will get a residue in the paper
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KA_P
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#8
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#8
(Original post by Nyx:))
So I'm revising chemistry and I have a couple questions I'm stuck on.
Without any visual indicators, how do you prove that a sample was partially soluble after putting it into the solvent(water)?

Why does every magnesium ion have two hydroxide ions in Mg(OH)2?

Your help would be greatly appreciated!
Not sure if it counts as a visual indicator but you could use a colorimeter.
Mg is 2+
OH is 1-
Mg(OH)2 has a charge of 0
You need 2 OH- to balance the Mg2+
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Nyx:)
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#9
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(Original post by scimus63)
Hi,
Your reasoning is OK but I think neutralisation reactions generally go to completion, they are not equilibrium reactions. If you add HCl to magnesium hydroxide I think the reaction will go pretty much 100% to products, magnesium chloride and water.

You also do not include solids in equilibrium constants- you cannot measure the concentration of a solid, like an ion or a gas in an equilibrium expression.

If this was an equilibrium reaction then what you say is good!

try here it might help:
https://science-revision.co.uk/A_Lev...constants.html

are you in y12?
Thanks for the reply! Hmm but the question states "an equilibrium exists between the salt and the ions after magnesium hydroxide is dissolved in h2o" so this equation is the equilibrium, doesn't that make it an equilibrium reaction?

Yes I'm in y12
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username5050312
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#10
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#10
(Original post by bl0bf1sh)
what is the charge of a magnesium ion? what is the charge of a hydroxide ion?
in order for the Mg(OH)2 compound to have no overall charge, what must the ratio of magnesium ions to hydroxide ions be?
Why is your pfp Donald Trump?
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scimus63
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#11
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#11
(Original post by Nyx:))
Yes I'm in y12
yes, but it wont be an equilibrium if you then add an acid to it, I could be wrong but I think neutralisation are not usually equilibrium reactions, they go to completion.

Dissolving the magnesium hydroxide solid in water will be an equilibrium reaction tho
Thanks for the reply! Hmm but the question states "an equilibrium exists between the salt and the ions after magnesium hydroxide is dissolved in h2o" so this equation is the equilibrium, doesn't that make it an equilibrium reaction?
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bl0bf1sh
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#12
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#12
(Original post by cleveranimal56)
Why is your pfp Donald Trump?
it's a blobfish, they do look pretty similar tbh
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KA_P
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#13
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#13
(Original post by cleveranimal56)
Why is your pfp Donald Trump?
I thought it was a fish...
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Nyx:)
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#14
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#14
(Original post by scimus63)
yes, but it wont be an equilibrium if you then add an acid to it, I could be wrong but I think neutralisation are not usually equilibrium reactions, they go to completion.

Dissolving the magnesium hydroxide solid in water will be an equilibrium reaction tho
Thanks for the reply! Hmm but the question states "an equilibrium exists between the salt and the ions after magnesium hydroxide is dissolved in h2o" so this equation is the equilibrium, doesn't that make it an equilibrium reaction?
Oh alright, makes sense!
If I write a kc equation for only the equalibrium reaction, should it involve the charges or no? Like kc= [Mg2+] [OH-]^2
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scimus63
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#15
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#15
you can put them in if you like, I would
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