Which of the following compounds has the highest oxygen content by weight?

The molecular weight is 18, and 16 of that is oxygen.  

So, water is 16/18 * 100 = 88.9% oxygen by weight.  

None of the other compounds have as high an oxygen content (by weight) as water.  

This is a trick question, and the University Chemistry grad fell for it.  

If you have the same weight of each compound (let's say, 10 grams, for example), then H2O has the most oxygen weight (8.89 g).

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.