Water of crystallisation

[Romix]

Hello dear forum members, here's the question for you.

 

CuSO4 + 5H2O ---> CuSO4.5H20

Where this 5 came from?

How u work out that it's 5?

 

Na2SO4 + 10H20 ---> Na2SO4.10H20

Same question here, why there's ten H2O?

Edited September 7, 2014 by Romix

[fiveworlds]

CuSO4 + 5H2O ---> CuSO4.5H20

Where this 5 came from?

How u work out that it's 5?

 

It reads 5 molecules of water plus 1 molecule of Copper Sulfate gives 1 molecule of Sulfate Pentahydrate.

--->

 

Should include some reference to temperature/pressure for the reaction to occur. Which can be omitted.

Na2SO4 + 10H20 ---> Na2SO4.10H20

 

1 Sodium Sulfate molecule plus 10 water molecules gives 1 molecule of Sodium Sulfate decahydrate http://www.sigmaaldrich.com/catalog/product/sial/403008?lang=en&region=IE

 

These decas(10) and (5)pentas http://en.wiktionary.org/wiki/penta- are numbers in latin/Greek. There is also (8)Octa (4)Tetra (2)di (3)tri (6)exa/hexa (7)hepta

Edited September 6, 2014 by fiveworlds

[Romix]

Ok, thanks

[Enthalpy]

Hi Romix, welcome here!

 

5 or 10 because the crystal wants it that way. I can't imagine a way to predict that: it must depend on crystal packing, nothing related with valences or simple notions.

 

I've seen variable amounts of crystallisation water, depending on the compound history; I can't tell for sure if, at small scale, there a few possible numbers which average out over many crystals to make a continuously variable composition, or if the number can vary in unit amount locally.

 

For gem crystals, books indicate a uniform and integer number a water molecules per cell; this favours the first hypothesis.

[ajkoer]

To quote from Wikipedia (http://en.m.wikipedia.org/wiki/Water_of_crystallisation#undefined ):

 

"Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal cation."

 

So per Wikipedia, the water molecule in the formed crystal structure that are not directly bonded to metal cation (usually containing a +2 and +3 cations as well as a −2 anions) are usually termed "water of crystallization".

 

Now, in the case of copper(II) sulfate, it is more actually represented as [Cu(H2O)4]SO4·H2O. So, only one molecular of water is technically a "water of crystallization", the other four are coordinated to the Copper cation.

 

For Glauber's salt, Na2SO4(H2O)10, there is technically no water of crystallization.

 

For Epsom salts, I recall seeing it express as MgSO4(H2O)6.H2O implying a single water of crystallization. In fact, for reason of accessibility most likely, if you need to extract water from say isopropyl alcohol, to make it drier, using MgSO4 is often recommended (I have done it with some success). If I am correct here, the recovered hydrated salt after being dried and heated, could liberate some alcohol per the single water of crystallization, which I would describe as in effect, solution water (a mix of water and alcohol, in the current case).

Edited October 5, 2014 by ajkoer

[John Cuthber]

I don't know where Wiki got that definition, but classically, it was impossible to know if the water was bound directly to the cation or not.

So the definition can not be right.

All the water was counted as water of crystallisation.