[Waldo]

I have a problem to solve and don't know how to do it :

A quantity of 30g of a chromium oxide reacts with hydrogen at high temperature and produces 14,6 g of water and chromium (pure).
The "yield" of the reaction is 90%.
The question is : find the chemical formula of this oxide.

Can someone help me ? 

Thank you

[FeLiXe]

you could write down the balanced equation where the chromium oxide is CrO_x. then you do typical stochiometry

if you don't like that just think about how many moles 14.6g of water are. notice that they contain 90% of the oxygen that was originally in the chromium oxide.

[AWK]

The serious problem exist - chromium cannot be pure when yield of reaction is 90%, or you use a mixture of chromium and chromium oxide for reduction. The other possibility is you catch only 90 % of water.