Sciencemadness Discussion Board

Everyone's selling water (hydrates)

Fyndium - 17-10-2020 at 00:49

I can't say I just realized it, but I noticed that when I bought washing soda, thinking it was better for chemistry as being sodium carbonate, I noticed that it actually contains 10 molecules of water, hence the total molar weight is 286g as decahydrate, and only 106g as anhydrous. So, basically a package costing around 5 euros suddenly turns into 10 euros for water and 5 euros for the actual stuff if you want a full 1kg of it. Hence it actually is much more economical to buy sodium bicarbonate and thermally decompose it for the use, since it appears even cheaper and the weight is reduced much less.

Same applies to multitude of other reagents, like common desiccants. There is an exemplary video on tube where 1kg of magnesium sulfate is reduced to less than a half of it's weight, including the added cost of one 2L flask. It is very annoying to carry several kg of plain water and pay big buck for it instead of getting the pure compound.

This is a lesson that a very small detail can cut yields by over 70% because for example in metathesis reactions the hydrate not only reduces the amount of available molecule but also dilutes the solution. Some synthesis lack the notion of which form is used, but thankfully many times the molar weight is stated so one can check if it means anhydrous or hydrate.

mackolol - 17-10-2020 at 02:52

Yeah... I have the opposite problem. I wanted to make cooling mixture, by mixing ice with CaCl2, unfortunately it only works with hydrate as water absorbtion is exothermic reaction. And its hydrate is uncommon to buy and much more expensive in my country.
And after all, if you want to have hydrate, but only have ahydrous salt, you have to mess around with it (dissolve it in water, then distill and freeze out the crystals yet with some loss).
But when you buy the hydrate, you have a ready product of the salt reacted with water. And as you may know, hydrates dissolve in water much more quickly, which makes them much more convenient to work with, since probably, you're gonna dissolve them in water anyways. So I wouldn't call the product of salt and water just selling water, because if you buy for example H2SO4, you don't call it paying for water although it is the substrate.

[Edited on 17-10-2020 by mackolol]

unionised - 17-10-2020 at 03:28

It's actually worse than the OP suggests.
For example I wanted some freshly precipitated magnesium carbonate.
The "obvious" method is from mag sulphate and washing soda.
But, on reflection, I realised I didn't really want sodium or sulphate.
So, in addition to buying a lot of water of crystallization, I also had to buy some unwanted counter-ions.

Surely someone should be able to sell me some carbonate ions without having to pay for all that sodium?

wg48temp9 - 17-10-2020 at 04:50

Quote: Originally posted by unionised  
It's actually worse than the OP suggests.
For example I wanted some freshly precipitated magnesium carbonate.
The "obvious" method is from mag sulphate and washing soda.
But, on reflection, I realised I didn't really want sodium or sulphate.
So, in addition to buying a lot of water of crystallization, I also had to buy some unwanted counter-ions.

Surely someone should be able to sell me some carbonate ions without having to pay for all that sodium?


Yes but they would need to be kept in an ion trap and you would get even less than a few pico grams LOL