[nuevaquimica]

Hello,
I'm trying to make a 1M concentration of MgCl2 but I only have anhydrous MgCl2. Is there a safe way to make this or should I just wait for the hexahydrate form?

[Arkcon]

Yes, you can make solutions with anhydrous forms of salts.  There are a few caveats.  You guess correctly, the hydration may be energetic.  Shouldn't be too much of a problem, just add water a little at a time, maybe make sure the beaker is much larger than typical. 

Often the anhydrous form isn't soluble in water.  That's not a joke, the powder won't dissolve, you have to hydrate first, then dissolve.  So you want to be sure you turn all the the grey, fluffy, MgCl2 into boxy clear crystals.  Then dissolve, then QS to correct volume.

I hope you're able to use the formula weight of anhydrous salt, and aren't relying on a recipe written for the hydrous salt. Because in that case, the weight will be off.

And if you open the jar, and its partly crystallized already, then just forget about it.  You won't know how much is hydrated already and how much isn't.

[Borek]

just add water a little at a time

I think you meant "just add TO water a little at a time". In any way, adding to water is a much safer approach.

[Arkcon]

Really?  I that really very important?  This isn't mixing of conc. sulfuric, which can splash dangerously.  This just as I described:  a white fluffy powder, that re-hydrates, forming the bulky clear crystals associated with Epsom salts.  Gets a little warm, maybe, but its not going to jump out of a beaker.  Or is it?  And if so, wouldn't sprinkling a fluffy, light energetic powder, on a bulk of water be worse, if it floats?

[Borek]

Really?  I that really very important?

Very? Definitely not. But it is one of these things I consider "a better habit" in general, especially as it doesn't require more effort. Now and then reactions between water and solids are energetic enough you can hear hissing after adding water (I am not saying this is specifically case with anhydrous MgCl₂), so there is a chance some nasty vapors are produced. If you add solid to water heat is dispersed much faster and is not as localized.

[Arkcon]

I guess its the same rule, add to water to insure you have a bulk of thermally absorbing material around.

[Babcock_Hall]

There is one other consideration which suggests waiting for the hexahydrate to arrive might not be such a bad idea, especially if it has already been ordered.  The anhydrous salt can be used as a drying agent for organic liquids, but the hexahydrate cannot.  This might or might not be an important consideration for your lab.