[kleiss]

Hi,
I am a Bio Teacher that is teaching Chem this semester.  I have to practice certain labs for the performance based assessment (PBA) for the class. I was looking at doing the Magnesium Sulfate hydration lab. What is the purpose of putting the heated crucible of MgSO4 in a desiccator as opposed to letting the crucible cool on the triangle before taking the mass?  I have one, but it is old and very small. It would be somewhat difficult for 16 crucibles to fit in there without getting broke.
Thanks for the help
kleiss

[AWK]

What is the purpose of putting the heated crucible of MgSO4 in a desiccator as opposed to letting the crucible cool on the triangle before taking the mass?  I have one, but it is old and very small. It would be somewhat difficult for 16 crucibles to fit in there without getting broke.

Anhydrous MgSO4 is used as dessicant.
 Why not put a half of crucibles into  a dessicator and compare results?

[kleiss]

Thanks for the idea!  The dessicator is fairly old. I don't think it has a vaccum seal in it and I think it has some old Calcium chloride in the bottom which needs replacing. What other chemicals can I use in the bottom? Would these factors really make a significant difference in the masses?  The kids did it today and they got fairly consistent results with 2 heatings with letting it cool on a ceramic tile.  But, they haven't yet calculated the percent error of the lab. But, anytime they can light a fire, they like it.  I appreciate the help. Thanks.
kjkleiss

[Alpha-Omega]

MgSO4 is dessicant and if you allow a crucible of it to cool on that triangle it will absorb moisture from the air.  If it is a dessicant it absorbs moisture/water...so you put the hot crucible in a dessicator to keep it from absorbing moisture.

If you alllow that to cool in the open the mass will be inaccurate...it will have absorbed moisture from the atmosphere/surroundings.....You put it in a dessicator to cool so it will be free of moisture upon cooling.  Then when you weight it you get an accurate weight.

Dessicants absorb moisture/water.....you can try reactivating your old calcium chloride by drying it in an oven overnight at 110 degrees C.

[kleiss]

Thanks for the *delete me*!! I'll be baking some calcium tonight instead of cookies!!
Kleiss

[Alpha-Omega]

FYI:  Not only does the MgSO4 absorb moisture...so do the crucibles.  That is why crucibles are always brought to constant weight prior to gravametric analysis.

You wash them and dry them quantitaivlely.  Dry in a 110 C oven for 3 hrs...then put them in muffle furnace at between 650 and 850 overnight...you store them in a dessicator so they do not absorb moisture....when you do that...the seal on the descicator (vacuum grease on that lid)...you have to pop it...so it does not vacuum seal from the heat contained inside....that is so you do not wind up breaking the lid...

Everything for your experiment must be kept in a descicator....OK!!!!!