From a laboratory process designed to separate water into hydrogen and oxygen gas, a student collected 10.2 g of hydrogen and 66.9 g of oxygen. How much water was originally involved in the process?
Well, I suppose mass was conserved
10.2 + 66.9 = 77.1
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You can stop there.
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Now let's check if they are in the right ratio
2 H2O ---> 2 H2 + O2
2 mols of H2 for every mol of O2
mol of H2 has mass 2 g
mol O2 has mass 32 g
so it should be 4/32 = 1/8
Well, it does not check :(