Copper and chlorine form compounds at 0.558 g and 1.116 g of chlorine per gram of copper, a ratio of:

1. The law of multiple proportions compares two compounds of the same pair of elements. 2. Copper and chlorine form a green crystalline solid with a mass ratio of 0.558 g chlorine to 1 g copper. 3. They also form a brown crystalline solid with a mass ratio of 1.116 g chlorine to 1 g copper. 4. Dividing 1.116 by 0.558 gives exactly two, so the ratio of the ratios is one to two. 5. Taking a ratio of these ratios is what yields the useful small whole number. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 2 'Atoms, Molecules, and Ions', sections 2.1-2.6_