# CHEMISTRY

posted by cazzy

Do you think it is more useful to have the percent by mass of water in hydrate or the percentage composition, assuming that you know the formula for the associated dehydrated compound?

Can someone please explain what's the meaning of this question, I'm confuse. Thanks!

1. DrBob222

I believe would rather have the percent by mass in the hydrate if I know the formula of the dehydrated material. For example, if I know Na2SO4.xH2O is 55.9% H2O, then I know Na2SO4 is 44.1%.
I can take 100 g sample which gives
44.1 g Na2SO4 or 44.1/142 = 0.310 mols Na2SO4.
55.9 g H2O or 55.9/18 = 3.10 mols H2O
which is clearly 1 mol Na2SO4 to 10 mols H2O for the formula of Na2SO4.10H2O

2. mitchel

I already computed for the mass and moles of the hydrated salt which is CuSO4 .5H2O,
I just don't understand the question, whether i will choose which method then explain why??

3. DrBob222

I think the question is asking
a. If you know the formula is CuSO4 for the dehydrate, would you rather have the % H2O (as my calculation above) for you to determine the formula of the hydrate
OR
b. would you rather have the percent composition; i.e.,
%Cu
%S
%O
%H
I think if you choose b you must find a way to apportion the %O between the SO4 part and the H2O part. That can be done, of course, knowing the formula of the dehydrate; but I think a is the simpler method.

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