copper oxide is only composed of copper and oxygen
mass of copper oxide is 4.21 grams
mass of copper is 3.74 grams
substracting the 2 values we know the mass of oxygen is 0.47 gram
the ratio of copper to oxygen can be found by comparing the mole of copper against mole of oxygen
mole of copper = 3.47 / 63.5 = 0.0589
mole of oxygen = 0.47 / 16 = 0.0294
copper: oxygen is roughly 2:1
hence the formula of the copper oxide is Cu2O
the reaction will be
Cu2O + H2 —> 2Cu + H2O
