Hydrogen was passed over heated 2g copper oxide till only copper was left. The mass of copper obtained was was 1.6 grams. What is the percentage of oxygen in copper oxide?
There is a loss of 0.4 grams.
The Hydrogen will combine with the Oxygen in the Copper Oxide to form water (Hydrogen Oxide) removing the Oxygen form the Copper Oxide.
The loss of 0.4 grams is the loss of Oxygen from the Copper Oxide compound. Therefore there were 0.4 grams of Oxygen in the compound out of the original 2.0 grams.