crystallization water in solution prep.

[mkarjala76]

Hi,

Should the crystallization water be taken into account in the preparation of solutions?

For example M(MgCl2*6H2O) is 203.301 g/mol where only 47% comes from MgCl2 and rest from the six crystallization waters. So should you be: dissolving 203.301 g or 434.15 g MgCl2*6H2O per litre?

thanks

[mkarjala76]

...For 1M solution

[RobJim]

If you know that your solute is definitely complexed to six water molecules per molecule, then include the mass of the H2O atoms when weighing it. Another option is to carefully dry the solute until you're sure the water is removed and then to weigh it out without including the mass of the H2O molecules.

[mkarjala76]

no. I figured this out myself right after posting here (as usual, only asking or writing down the problem helps to solve it)

If you weigh 203.301 g = 1 mol of MgCl2*6H2O you are weighing 1 mol of MgCl2 at the same time cause mole is the number of molecules. Crystallization water doesn't matter cause it dissolves in when you fill up. Preparing a g/L -solution would be a different story.

thanks anyway