[Samuel Johnson]

Hi all, I have some Ferric chloride hexahydrate crystals, can someone tell me how much water I need to add to make a 29% solution, thank you in advance, any advice on how you mix it ?

[Arkcon]

That really depends on a number of factors, depending on what you really need.  A recipe, for your particular application, is probably the best bet.  However, let's try:

As a simple example, if something were '10%', what does that mean?  If you have 100 marbles, and 10% are black, and the rest are white?  What actual number is there in each case?   Your problem isn't too much different, but adapting to different units ... that makes the question difficult.

You need to find the recipe for your application.

[Borek]

Actually it is a bit more difficult as FeCl₃·6H₂O already contains 40% water.

[Arkcon]

Yes.  But that's even a hidden part of the problem.  If the recipe, for a stain or finishing solution or some such, calls for 29% solution, where does that number come from?  Is a 17% solution of iron chloride what's needed, and they just asked for 29% to compensate for the water of crystallization?  Then we know the recipe is calling for mass per volume ... or do we?