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).

"you start with 15.67 g mgco3-xh2o, when heated and all the water is removed, you are left with 7.58 g mgco3 only.

therefore, you had 15.67 - 7.58 = 8.09g h2o  

moles of each :

7.58g mgco3 = ? moles = 7.58g / 83.4g/mole = 0.09moles  

8 09g h2o = ? moles = 8.09g / 18g/mole = 0.44moles  

molar ratio between the two by dividing both numbers by the smallest number :

0.09 / 0.09 = 1  

0.44 / 0.09 = 4.8  

mgco3-5h2o"