I guess to sum up, If the dihydrate's going cheap, then go for it. Me being the way I am, I'd go for the kilo of lab grade 
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A little bit of chemistry formula fun for anyone who may be interested:
number of moles (unit = mole) = mass (unit = grams) / relative molecular mass (unit = grams per mole)
or n = m / Mr
A mole (aside from being a little furry critter) is a unit for the "amount of stuff", so by knowing the relative molecular mass of one form, lets us work out how much of another (in grams) we'd need.
Relative molecular masses can usually be obtained by wiki or a database, but can be got by just adding up how many atoms of a certain type are in the molecule you're looking at, then multiplying by the relative atomic mass (which can be obtained from the good ol' periodic table) and summing up the values.
e.g. for CaCl2.2H2O, Mr = 40 + (2 x 35.5) + (4 x 1) + (2 x 16) = 147 g/mol (atomic mass of calcium is ~40 and that of chlorine is ~35.5, Hydrogen ~1 and oxygen ~16)
Doing this:
Mr of anhydrous = 111 g/mol
Mr of dihydrate = 147 g/mol
Mr of hexahydrate = 219 g/mol
If we say that we have 1 gram of the dihydrate, then we can work out that we have (1g) / (147g/mol) = 0.0068 moles of calcium chloride dihydrate present (that's around four thousand one hundred billion billion molecules :grin:). Rearranging the formula to get mass, and plugging in the relative molecular mass for the hexahydrate, we get 0.0068 mol * 219 g/mol = 1.489 grams (so 1.5 g). This means that for every gram of dihydrate that is needed, you can throw in 1.5g of hexahydrate instead and it'll contain the same amount of calcium ions (and chloride).
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tl;dr version: It's all calcium chloride, just with more or less water associated. If you're more comfortable sticking with the dihydrate, then you're probably very sensible. If you like a bit of maths and want to do some calculations, here's how. That's made me thirsty, time for a pint.
----------------------------------------------------------------------------
A little bit of chemistry formula fun for anyone who may be interested:
number of moles (unit = mole) = mass (unit = grams) / relative molecular mass (unit = grams per mole)
or n = m / Mr
A mole (aside from being a little furry critter) is a unit for the "amount of stuff", so by knowing the relative molecular mass of one form, lets us work out how much of another (in grams) we'd need.
Relative molecular masses can usually be obtained by wiki or a database, but can be got by just adding up how many atoms of a certain type are in the molecule you're looking at, then multiplying by the relative atomic mass (which can be obtained from the good ol' periodic table) and summing up the values.
e.g. for CaCl2.2H2O, Mr = 40 + (2 x 35.5) + (4 x 1) + (2 x 16) = 147 g/mol (atomic mass of calcium is ~40 and that of chlorine is ~35.5, Hydrogen ~1 and oxygen ~16)
Doing this:
Mr of anhydrous = 111 g/mol
Mr of dihydrate = 147 g/mol
Mr of hexahydrate = 219 g/mol
If we say that we have 1 gram of the dihydrate, then we can work out that we have (1g) / (147g/mol) = 0.0068 moles of calcium chloride dihydrate present (that's around four thousand one hundred billion billion molecules :grin:). Rearranging the formula to get mass, and plugging in the relative molecular mass for the hexahydrate, we get 0.0068 mol * 219 g/mol = 1.489 grams (so 1.5 g). This means that for every gram of dihydrate that is needed, you can throw in 1.5g of hexahydrate instead and it'll contain the same amount of calcium ions (and chloride).
----------------------------------------------------------------------------
tl;dr version: It's all calcium chloride, just with more or less water associated. If you're more comfortable sticking with the dihydrate, then you're probably very sensible. If you like a bit of maths and want to do some calculations, here's how
. That's made me thirsty, time for a pint.
