When copper (cu) and oxygen (o) combine, two different compounds are formed. the first compound contains 88.8% copper by mass, and the second compound contains 79.9% copper by mass. (note: assume a 100-gram sample of each compound.) (a) show that cu and o combine with each other in conformity with the law of multiple proportions. the small whole number ratio of the mass of cu per gram of o in the first compound to the mass of cu per gram of o in the second compound equals : 1. this ratio indicates that cu and o with each other in conformity with the law of multiple proportions.