MadSci Network: Chemistry

Re: help needed on solubility

Date: Mon Sep 20 20:05:24 1999
Posted By: Greta Hardin, Secondary School Teacher, Science
Area of science: Chemistry
ID: 937545305.Ch

You are halfway there... you are looking down all the right paths.

Step one: get your hands on a regular chem book and/or the _CRC handbook  of 
Physics and Chemistry_.  Both have tables of Ksp.  In the chem book look  
under Ksp. In the CRC, look up "solubility product constants" (which is what 
Ksp stands for) and "solubility salts, chart or in selected solvents".  I  
checked the Ksp table real quick in my edition of the CRC... some of your 
salts aren't in there.  This makes sense however,  since Ksp's are usually 
only calculated for slightly soluble salts and if any of your salts are 
better than "slightly soluble" they probobly don't have a Ksp.

Step two: Look for those compounds in the CRC in the big table of Physical 
Constants of Inorganic Compounds... you will find all the information.  This 
is in the front of the "B" section of the CRC.  If you've never used a CRC,  
just open it up and start browsing.  The page numbers will start to make 

Anyway,  once you've found the Physical Constants of all your compounds... 
especially "solubilty in grams per 100 cc" (which translates to 'how many 
grams dissolve in 100 ml of water) compare "cold water" to "hot water."  
This will help you figure out whether their solubility increases with the 
water temperature or decreases.

Generally, the warmer the water, the better a solid dissolves,  and the 
worse a gas dissolves.  But I  will emphasize this is only USUALLY true.  In 
fact I spot one exception to that rule in your list right away.

I will walk you through a compound not in your list to give you an example 
to work with:

MnSO4 - 
in the cold water column there is a "52" with a little "5" superscript.  
This means that 52g of MnSO4 dissolve in 5 degree celcius water.

in the hot water column there is a "70" with a little "70" superscript.  
This means that 70g of MnSO4 dissolve in 70 degree C water.

That means that as the water gets warmer,  more manganese (II) sulfate 

If you were to go by Ksp, you would compare Ksp's of the different 
chemicals.  The smaller the number (the more negative the exponent) the less 
soluble something is.  Ksp's in tables are usually only at 25 deg. C and so 
would not be helpful for determining different solubilities at different 
temperature... but they are helpful in determining comparative solubilities 
at a single temperature.

I'm not going to go into an explanation of Ksp here... becuase I'm not sure 
if you needed one.  Further,  I  am afraid a quickly typed explanation might 
confuse more than it helps.  If this is a science assignment, your teacher 
sounds like one sharp cookie - and could probobly give you any more 
expalnation regarding Ksp that you need.

Good Luck!  I hope this helps.  Oh yes, if you can't get your hands on a CRC 
at school ask at the reference desk of your school or local library.  Both 
really should have one.

Greta Hardin

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