Question

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.

Answer 1

1) Chemical reaction 1: 4Cu + O₂ →  2Cu₂O.
n(Cu) = 88,8 ÷ 63,55.
n(Cu) = 1,4.
n(O) = 11,2 ÷ 16.
n(O) = 0,7.
n(Cu) : n(O) = 1,4 : 0,7.
n(Cu) : n(O) = 2 : 1.
Compound is Cu₂O.
2) Chemical reaction 2: 2Cu + O₂ →  2CuO.
n(Cu) = 79,9 ÷ 63,55.
n(Cu) = 1,257.
n(O) = 20,1 ÷ 16.
n(O) = 1,257.
n(Cu) : n(O) = 1,257 : 1,257.
n(Cu) : n(O) = 1 : 1.
Compound is CuO.