hey anyone know an easy way of learning the solubility rules???

[an1423]

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Its just so annoying and I always seem to forget them

 

[doc3232]

Considering how many people get perfect scores on G-Chem, I wonder if we need to know them...

 

[soopasteve]

Its just so annoying and I always seem to forget them

one bit of advice I can give you is to consciously seperate the difference between solubility and melting/boiling points. I tutor and one of the biggest hangups with alot of people is getting the characteristics mixed up and it causes problems. If you consciously step aside and realize theyre entirely different properties then the characteristics that define them will be a bit more digestible. Some things to note;

1) if a molecule has both a hydrophilic and a hydrophobic region, the larger the hydrophobic region the less soluble the compound will be. people get this mixed up with increasing melting/boiling points due to weight and dispersion forces. on the other hand, the larger the hydrophillic region, the more soluble the molecule will be.

2) like solvates like

3) if you're dealing with a molecule that is polar, it WILL benefit from hydrogen bonding without actually having an OH,NH,SH hydrogens. it will not be a 2-way relationship like those, but a one way relationship that still helps solubility.

if you provide specific questions or vague spots I would be more than happy to explain them. theres alot of info on solvents/solutes that it would be too much to bother typing up when a good portion of it you probably already have a grasp on. ill keep track of this thread incase you respond with specific examples of where you are getting stuck.

 

[Contach]

**** eh. this gets asked like every week. do a search.
here's a mnemonic we stole from the pre-med forum. I made my own modification and here it is:

CASH'N GIA. The story is: Gia is my hoe, and she owes me money because I'm a pimp. I was cash'n Gia but was caught HAPpily on CBS.

C - chlorides and halogens except HAP (heavy metals: Ag, Hg, Pb)
A - ammonium and acetates
S - sulfates except CBS (Ca, Ba, Sr)
H - hydroxides and oxides with CBS (Ca, Ba, Sr)
N - nitrates
Gia - group 1 alkali metals (Li, Na, K, etc

f this garbage. it better get me atleast 1 point on the DAT or I will have some REAL rage to deal with.

These are all the salts that are soluble in water. For our purposes I think of this as 100% solubility - but this is not technically true.
Note: solubility is not an absolute, it is on a continuum. Some books will say slightly different b/c they draw the line somewhere else.

 

[Contach]

1) if a molecule has both a hydrophilic and a hydrophobic region, the larger the hydrophobic region the less soluble the compound will be. people get this mixed up with increasing melting/boiling points due to weight and dispersion forces. on the other hand, the larger the hydrophillic region, the more soluble the molecule will be.

3) if you're dealing with a molecule that is polar, it WILL benefit from hydrogen bonding without actually having an OH,NH,SH hydrogens. it will not be a 2-way relationship like those, but a one way relationship that still helps solubility.

hi soopasteve. thanks for your kind post and willingness to help. sorry to hijack OP's thread.

Question 1)
So I understand taht boiling point/melting point or organic compounds is based on these three things:
1) Presence of hydrogen bonding (of course ionic would be higher)
2) Presence of dipole-dipole interactions
3) Molecular weight (higher molecular weight = higher mp/bp)
4) Branching (more branching = lower mp/bp)

This is for the most part true. But where I get really confused is when they make the distinction between cis- and trans- geometric isomers' mp and bp. One they say has a higher mp.. I think its cis- because it has better dipole-dipole forces and the trans- has a higher bp than cis- because it cannot pack correctly.

this throws everything off for me. Can you explain why the relative bp and mp is different for these compounds? whereas every other compound has a bp proportional to its mp?

Question 2)
Why does hydrogen bonding increase solubility? I saw this in some of my kaplan material, and it seems really counterintuitive. Like you said, it's good to think of mp/bp and solubility as two separate things, but here it gets mixed up.

Question 3)
Is there such thing as intramolecular Hydrogen bonding?

Question 4)
Is acetone soluble in water?

Question 5)
What's the difference between miscibility and solubility?

thanks - these have been bugging me for a while.

 

[admap]

There's one little mistake, the B in CBS is Ba, not Br. Right?

 

[Contach]

There's one little mistake, the B in CBS is Ba, not Br. Right?

yueah sorry.
calcium, barium, strontium
ca ba sr

i usually just look at the periodic and they are the 3 in that row.
thanks. ill edit that.

 

[admap]

Yeah I know what you're saying..!! I tend to make silly mistakes like that on practice tests...Hope I don't do that for my actual DAT, which is in a day!!!

 

[soopasteve]

hi soopasteve. thanks for your kind post and willingness to help. sorry to hijack OP's thread.

Question 1)
So I understand taht boiling point/melting point or organic compounds is based on these three things:
1) Presence of hydrogen bonding (of course ionic would be higher)
2) Presence of dipole-dipole interactions
3) Molecular weight (higher molecular weight = higher mp/bp)
4) Branching (more branching = lower mp/bp)

This is for the most part true. But where I get really confused is when they make the distinction between cis- and trans- geometric isomers' mp and bp. One they say has a higher mp.. I think its cis- because it has better dipole-dipole forces and the trans- has a higher bp than cis- because it cannot pack correctly.

this throws everything off for me. Can you explain why the relative bp and mp is different for these compounds? whereas every other compound has a bp proportional to its mp?

Question 2)
Why does hydrogen bonding increase solubility? I saw this in some of my kaplan material, and it seems really counterintuitive. Like you said, it's good to think of mp/bp and solubility as two separate things, but here it gets mixed up.

Question 3)
Is there such thing as intramolecular Hydrogen bonding?

Question 4)
Is acetone soluble in water?

Question 5)
What's the difference between miscibility and solubility?

thanks - these have been bugging me for a while.

a few of your questions can be answered by correcting something that you said that is incorrect. branching actually affects mp and bp differently. branching will raise the mp due to the molecules being able to pack closer together. on the other hand it will lower the boiling point due to less surface area. this makes sense if you consider the fact that when something is already in liquid form and is nearing its boiling point, the packing isnt an issue since the molecules arent close enough to eachother and the only thing affecting them at that point is their charges. on the other hand, when considering a solid, efficient packing is preferential since we know bond strength is directly related to bond length.

Question 2)hydrogen bonding increases solubility (in water) due to being able to overpower the hydrogen bonds between water molecules. nonpolar molecules arent soluble in water since the water molecules are more attracted to themselves then to the nonpolar molecules. so, if a water molecule has a choice of forming a strong bond with itself or a weak bond with a nonpolar molecule, its going to go for the strong bond. thats why layers are formed with oil in water. water is so preferrential to itself that it actually pushes the oil's molecules out of its solution.

question 3)this could depend on your definition of molecule. a good example would be DNA as well as proteins. remember that a molecule of DNA is hydrogen bonded together down its base pairs. proteins are held in certain conformational shapes through hydrogen bonding as well. if you dont consider proteins/dna to be molecules (and instead consider their individual nucleotides and amino acids as molecules) then i would say no. i personally do consider them molecules (esp proteins) so my answer is yes.

question 4) yes, its actually miscible in water due to its relatively small size and electron congregation around the oxygen.

question 5) a miscible substance is a relative term to describe a substances ability to dissolve in any concentration inside a particular solution. aka: it isnt a universal term. for example, a molecule can be miscible in one substance and not in another. a substance that is considered soluble just implies its ability to form a homogenous mixture. usually substances are only solluble to a certain extent, and anything additional will settle at the bottom of the solution. thats where miscible comes in, if the substance is miscible then the concentration doesn't matter. picture being able to dump 2 pounds of sugar into your tea and having none of it settle at the bottom, that would be miscible. 😉 (by the way, sugar is NOT miscible in water, so dont try this, lol).

 

[triad027]

lol i memorized the whole table of hybridization stuff like the shapes and angles had 0 questions about it.

memorized solubility rules 0 questions.

and memorized the triple point graph etc 0 questions...

 

[Contach]

thanks soopasteve!

So increased branching means molecules can pack closer together, means they prefer the solid state, thus causing them to have a higher melting point. And since trans- geometric isomers can't pack as well as cis-, trans- will have a higher melting point. thus they will be solid more often than cis-. And this is why trans fats are bad for you - because they are solid and clog up arteries. Makes sense.
But boiling point does not depend on packing because the molecules are farther apart in solid. It is more dependent on intermolecular forces. Cis- will have net dipoles and thus have higher boiling points than trans. This also makes sense.

Ahh, miscibility makes sense now too.
Sweet.