When 10 grams of hydrogen gas is burned with oxygen gas, it produces 18 grams of water. This is a chemical change.

Question

Based upon conservation of mass, how much oxygen gass was used

Answer 1

Conservation of mass means that the mass of the reactants must be equal to (the same as) the mass of the products.

The balanced equation for this chemical reaction is 2H₂(g) + O₂(g) ==>2H₂O(l)

So, if ONLY 18 grams of H₂O is formed, then that must mean that there isn't enough H₂ to make more.  The H₂ and O₂ react in a mole ratio of 1 mole H₂ to 1/2 mole O₂ to make 1 mole H₂O.  18 g H₂O = 1 mole H₂O, so you would need 

1 mole H₂ + 1/2 O₂ to make 1 mole H₂O.  This is represented in grams as follows:

2 g H₂ + 16 g O₂ to make 18 g H₂O.

So, the answer is 16 grams of oxygen gas was used.  But it should be noted that not all of the hydrogen gas was used. There will still be 8 grams of H₂ gas left over.