When 1.375 g of copper (ii) oxide is reduced on heating in a current of hydrogen, the weight of copper remaining after the reaction is complete is 1.098 g. what is the percent composition of oxygen?

20.11% of oxygen

Explanation:

The reaction is:

2CuO ⇒ 2Cu + O2

If you have an initial amoung of a sample, thi may be have impurities, so you have to start from products to know what you had at the begining.

to make a relation between reagents and products you need to change the mass to mol, then you can relate them with the molar ratio.

Now you have the initial amount of oxygen, to convert to percent you must divide the mass of oxygen between the mass of the sample and multiply by 100