a student heats 2.005 g of a copper bromide hydrate of unknown molecular formula, completely driving off the water of hydration, and notices that the weight drops to 0.998g. Upon reduction with excess aluminum metal, the copper ions are reduced to 0.442 g of Cu (s). determine the formula of this unknown copper bromide hydrate

Answer 1

2.005 = mass CuxBry + H2O
0.998 = mass CuxBry
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1.007 = mass water

CuxBry + Al ==> Cu + ...not balanced.
0.998g.........0.442
moles Cu = 0.442/atomic mass Cu = about 0.007 but you need to be more exact.
So mols CuxBry must be 0.007

n = grams/molar mass and
molar mass = grams/n = 0.998/0.007 = about 143 or so for molar mass CuxBry.
143 = mass Cu + mass Br
-63.5 = mass Cu
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79.8 = approximate mass Br which is one Br so the formula must be CuBr.
Find moles CuBr (0.998/molar mass) = ?
mole H2O = 1.007/18 = ??
Now find the ratio H2O to 1 mole CuBr. The easy way to do this is to divide the smaller number by itself (which gives you 1.000) and divide the other number by the same small number. Round to whole number. That should give you x in CuBr.xH2O.